This is still the best answer in the thread.
Honorable mention.
I know I’m tipsy when I just have a “woo-hoo!” feeling about life - even at a funeral “party”
I know I’m drunk at the point when I take another sip and my stomach/brain tells me, through a flash of nausea, that I’ve gone too far. I stop drinking, get a little anxious, and switch to water.
My eyelids start to get warm and kind of heavy, which means I’m pretty buzzed. I know I’m really drunk when I want to be everyone’s best friend, I have no secrets, and I’ve kissed every girl in the room.
I pretty much loose my sense of smell as I pass from tipsy/buzzed to drunk. I’ll also catch myself dropping the F-bomb a lot.
Sort of a punch to the back of the head feeling followed by a head bob.
My social anxiety and self-consciousness disappear. The last time I got drunk I ended up trying to mediate an argument between a Puerto Rican woman and her Ecuadoran boyfriend on whether Puerto Ricans can relate to the ‘‘immigrant experience’’ as citizens of the U.S. It was a great conversation, but no way in hell would I have interjected my opinion on such a sensitive subject unless I was completely plastered.
I actually like myself better when I’m drunk.
There’s an iPod/iPhone App for that. That’s how I know, just like everything else. 
I stand up, and then say to myself “Oh, I’m drunk now.”
I’ve been told that I’m an extremely sober-seeming drunk person. My Dad’s that way as well, though he has the excuse of being an alcoholic
“So I guess you’re not really drinking tonight?”
“Well, I’ve had 10 beers…”
“:dubious:”
Like anything else, do you do something long enough, you get good at it.
I think it depends on what, exactly I drink. Tequila makes me do strange things.
That’s how I do it at Mexican restaurants. Standing up after sitting through a meal and 3 large-sized sweet drinks can be a revelation.
In that case, I have never been drunk. I always seem to have misplaced my shoes before I vomit on my stocking feet.
With grain alcohol (190 proof), you don’t know you’re drunk until you fall down - kinda takes the fun out of it.
I’m that way. Even when I drank regularly <socially; in a ski resort, there’s not much else to do at night> my experiences with alcohol would fluctuate wildly from night to night, everything from 3 Long Islands and I’m just fine to one wine cooler and I’m barfing.
Now I’m even less in control, since I drink-to-a-buzz maybe once every few years.
Tolerance = <0.
Being drunk as opposed to being sober is like the difference between Yankees and Damn Yakees; you either know it or you don’t. If you don’t, you are automatically unable to understand any explanation.
It doesn’t happen often, but if I start to hiccup while drinking, I know it’s time for bed (unfortunately, it will take a long time before I can get to sleep because it’s virtually impossible to get rid of the hiccup when I’m drunk).
When I feel like whatever I have to say is terribly important.
I get kind of an absent-minded clumsiness. I can be quite coordinated if I concentrate, but I tend to forget that there’s a glass right next to my hand before I knock it over, or that I started to pour my beer into another glass a few seconds ago and now, staring off into space, it’s time to stop pouring.
I also get kind of an odd sensation in my upper arms.
So THAT explains it! There must be a LOT of drunks around here! 
Room spinning is my gauge.