I prefer Dirk Gently, who is constantly haunted by the fact that his wild guesses and complete fabrications continue to unerringly be entirely true. 
A few months ago I correctly deduced something based solely on my knowledge of human nature. I’d been on vacation for a couple of weeks, and then the first week I came back there was a big event at work that included the staff from several different offices. At one point I noticed one of my immediate coworkers across the room talking to two women from one of the other offices, and saw these women react with excitement/joy and then both of them hugged my coworker.
I thought “She’s pregnant.” Then I thought, “Wait, how do I know that?” Well, there are only so many announcements that would produce that sort of reaction – I can’t imagine anything work related doing so – and she was already married, so it had to be pregnancy.
I did wonder why I hadn’t heard about this from my coworker herself, but I later realized she must have told everyone in our office while I was on vacation and forgot that I wouldn’t have been there. A week or two later she made some casual reference to being sick in the mornings and obviously expected that I’d understand what she was referring to and not be surprised. Thanks to my clever deduction, I did understand and was not surprised. 
I was going to say the same thing. I remember seeing one episode of Cops where the officer asked a normal-looking woman “when was the last time you used?” and shockingly, she answered! I’m sure that officer knows all the slight changes in appearance that drug users have.
Or, sometimes, as other said, it’s just deduction based on visible clues:
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There’s a house on my street with a husband, a wife, a teenage boy, and a younger boy.
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The husband had a pickup truck, the wife a sedan.
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About a year ago, another car appeared in the driveway. I’ve never seen in on the road, but it’s always in various states of either disassembly, or customization with chrome accessories and non-original headlights.
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The pickup truck is never parked there anymore. I have seen no replacement vehicle to take its place.
–Using that information, it’s pretty easy to deduct that the couple has separated or divorced, the man has left home, and the teenage son is at or near driving age, and is preparing his first car. I later heard another neighbor comment that the husband isn’t around anymore, so it turns out I was right.
–It’s also important to point out that I could have been wrong. Maybe the man of the family lost his license, or changed jobs to something accessible by public transportation, or simply missed his car payments. Just because we observe something and we’re 80% sure, we shouldn’t assume we’re always correct.
My point is, I came to these conclusions just by being alert and observant when I drive down my street, or when walking my dog at night. I don’t know these neighbors well, I don’t spy on them, and their life doesn’t interest me…it’s just something that entered my mind naturally.
My advice to the OP vanotd21 is, if you want your powers of deductive reasoning to be at their top form when they’re needed, you’ll want to practice them all the time. It costs nothing to keep a mental inventory of the cars that are supposed to be parked on your street, and gives you valuable free practice.
I always believed that Holmes was just a cold reader, and made up a bunch of stuff after the fact to explain stuff he’d figured out in other ways.
I knew a girl who is the daughter of a policeman, and she was really, really good at this sort of thing. She was a huge study of body language, not the stuff Holmes claims to have noticed, and yet makes similar predictions.
I think just anyone anywhere, including of course Holmes, could have a Holmonian moment. Its the constant parade of em that Holmes has that stretches the believability.
Of course the same can be said of Soap Operas, Sci-Fi series, Sitcoms and so on and so forth. A few epsisodes where this amazing/dramatic/important/only happens once in a lifetime thing occurs is of course reasonable. Most/all episodes being that way? Not likely. But, if they were much truer to real life nobody would be that interested in them.
You just have to be observant in other ways. Look at the shoes to start. Are they Tevos or flip flops on their feet, Nike shoes specifically made for tennis, or a knock off generic running show from Ross. Luggage is a give away; is the beat-up old bag leather with brass buckles, or just some piece of crap. A matched set of expensive newer luggage is probably nouveau riche, mismatched expensive luggage is from an established family. Is the T-Shirt topically ironic, moronic (One Tequila, two Tequila, Floor), or a random shirt picked off the rack. Are they carrying the newest cell phone, the one you get free with activation, or a Blackberry; those are all clues. Are they seasoned travelers that are comfortable in the airport (have water, a book, not too much carry-on, or are they burdened down with lots of crap and looking around nervously. How do they interact with airport staff? Deferential, confrontive, buddying up, or polite and firm. What are they drinking? If they are sunburnt they are coming from a short vacation. What sort of haircut do they have, what eyeglasses do they wear. There is still tons of information.
Reminds me of an old Holmes joke.
Holmes and Watson are camping. Late into the night Holmes wakes Watson and asks “What can you deduce from what you can see?” Watson observes he can see many many stars. And for all the stars he CAN see, there are probably many countless more that he can’t see. And this means nearly countless planets. And Earth is planet that has life. So, there is a high probability that there is other life out there.
Holmes “Very good work dear Watson. Do you know what I deduce?”
Watson “What?”
Holmes “Somebody stole our tent while we were sleeping”.
In The Adventure of the Blue Carbuncle, Holmes makes several deductions about the owner of a hat from examining the hat alone.
[QUOTE]
There’s a funny story by Stephen King where Watson solves a case while Holmes is sick.
See “The Yellow Face.” Holmes’s deductions are spectacularly wrong, but the case is still solved.
Holmes also admits to making incorrect deductions at certain points in The Valley of Fear and “The Bruce Partington Plans.” (I’m sure there are others, but those are the only ones that come to mind.)
Doyle also wrote a pastiche of his own work called How Watson Learned the Trick in which Watson, in attempting to emulate Holmes, makes a string of very wrong deductions.
I think there is a lot more to the cold reading comments. If you tell yourself, I don’t know the answer it is a lot harder to come to a conclusion. Now if you make a guess/hypothesis you start to look for things that support or break that guess. And then people only remember the big “hits” you have.
As an example, at a New England university, I had a summer research student from Texas A&M working under my tutelage. Before arriving, she emailed me that she would be flying a day late from Minnesota from an anniversary party for her grandparents. During the course of the summer, I would guess things about her randomly as a game. It started with, “So is your Dad upset that you cheer for the Rangers instead of the Twins?” Dead-on. So her Dad is from Minnesota and she is a bit rebelious (and from Texas as well). She is absolutely brilliant and going to a state school in Texas but applied for internship in Ivy League institution- “So J, are both of your parents professors at Texas A&M?” (one professor and one administrator).
Other correct statements I just guessed and put out there:
How was Space Camp?
Did your girl scout troop go camping a lot?
Was it hard playing oboe in the marching band? *My personally proudest one
But things I asked that were completely wrong but she never remembers when telling about my ability:
So your little sister likes art and english? (no sister)
Was it hard going to college a year early?
So how old will you be when you get your driver’s license?
A famous story about Bell told how Bell would teach courses in how to observe patient symptoms and how the doctor had to be totally observant. One of his teaching gimmicks was to dip a finger into a urine sample, then comment on the color of the urine, then comment on the smell and finally stick the finger in his mouth. He would then make the students do it and then point out that if they had been truly observant, they would have noticed that he stuck his second finger in the urine, but stuck his first finger in his mouth.
Apocryphal? Perhaps, but it’s a great story anyway. And would you expect anything less from Holmes’ role model?
OK, I can see a few ways to deduce having been in marching band (Holmes would probably be able to pick it out from gait alone, especially coupled with the person’s reaction to certain songs), but what was the basis for the oboe?