Just wondering. Its pretty easy for them to tell when we are. Do we have anyway of knowing?
They smell different, something like umm…alcohol.
Watch for garment shedding.
It’s very easy for me to tell when my wife is aroused. Every time a squardron of beavers flying ion drive equipped jump jets fly out of mozart’s resurrected ass harmoniously whistling Camptown Ladies my wife gets hot. Otherwise I can’t help you.
Soggy underwear. But you’d probably have a hard time checking that for yourself in a place like a bar. Try expensive jewelry instead.
Not to be the nerd in this post so far but someone already took my answer of losing items of clothing.
There are subtle signs of arousal…breasts enlarge (not a lot), cheeks flush, eyes dilate, nipples go erect and harder to check is lubrication and clitoral swelling. If you get to see that last part up close the question of arousal has been long answered anyway.
Some psychological signs might be licking of lips, playing with hair and laughing at your stupid jokes.
Women are always aroused.
Slightly off topic but related women also have distinct physilogocal responses to an orgasm. Too many women seem to think that Meg Ryan put the idea of faking an orgasm to rest in the movie When Harry Met Sally. If the man is paying attention to the woman all the, “Yes, YES, YESSSS!!! Ahhhhhhh!!” won’t fool him. Like a man women also have rhythmic contractions during orgasm. They also often ‘over lubricate’ (not sure what else to call it but some do it to the point of female ejaculation). You also might notice body tremors and hyper-sensitivity to touch immediately following her orgasm. Of course, every woman is different and not all orgasms are created equal but if you’re paying attention and especially if you know your female partner well you can twig to when the real thing happens and when she is faking with close to near certainty no matter how loudly she yells and moans.
Vasodilation and visible redistribution of blood.
Sometimes, if you’re paying attention, you’ll observe a bottom-to-top blush that’s for all the world like the graphic indicator that accompanies Jerry’s telethon.
Thanks for the real answers. I was worried the GQ would be turning into one of the other boards.
Only a little more alcohol and we’ll have enough to take it all the way! Keep those calls coming in! Next up, we have Paradise by the Dashboad Light by Meatloaf!
My favorite Meatloaf song. Definite classic for the ages!
Like I said, women are always aroused. Whether or not by you is a quesion I can’t answer. They’re normally highly aroused, much more when menstruating, less so in PMS but can be convinced with 15 second talking to.
As a woman, and an experienced people-watcher, I would also add that while, like the verbalization, body language can be faked, it’s a little tougher.
As Whack-a-mole pointed out, the licking of lips, and laughing at jokes she might otherwise not find funny, are both pretty clear. I play with my hair as a matter of habit, so I’m inclined to discount that as an indicator of anything beyond fidgeting, but that’s an individual thing.
Watch the hips, though. She who is horny tends to swing them just that eensy bit more than normal. I have noticed that she who is consciously putting on a show swings her hips as if she’s on a catwalk. There is a subtle difference, trust me. 
One thing I’ve noticed, and would like to follow up on, is that I become a great deal more aware of a woman’s perfume if she’s aroused. Several times, I have been out with the ladies, and inexplicably have become aware of Ms. X’s perfume-du-jour, only to study her and her behaviour, and determine that she was “in heat,” to be crass about it. (Caveat: these have been girlfiends of long standing, whom I know well. It would be a tricky indicator if you don’t know your lady.) 
For what it’s worth, the perfume thing also works with men and their cologne. If for no particular reason, you become aware of a companion’s scent (as opposed to BO), watch them. Odds are good they’re in the mood.
Getting crucial responses with dilation of the pupils
“Honey, get hip! It’s time to unzip, to unzip zip-a-zip, zip-a-zip.
Whippee!”
—Genesis, “Counting Out Time” from The Lamb Lies Down on Broadway
That’s a pretty broad brush there Not In Anger… I wouldn’t say we are always aroused. I know I am certainly not all the time and I’m at that stage of pregnancy where wanting some is a steady thing.
There is ‘playing with hair’ and ‘aroused playing with hair’. Many women I have known play with their hair to greater or lesser degrees. There is just something different in how they play with their hair when aroused as opposed to just fidgeting. I’m not sure I can really explain it but I think I can spot it when I see it even if I don’t know the woman well (ok…maybe give that a 50/50 chance if I’m honest about it…depends on circumstances to). More like preening than just yanking or twirling the hair. Again…hard to describe.
Not a lot?
When my wife’s turned on her boobs swell to almost double their already enormous size. This has continued to fascinate me for for 23+ years.
I must be doing something right!
Double?! Bottle whatever you’ve got and I’ll pay good money for it!
I’ve never noticed that spectacular a transition. I’ve seen the breasts of my female acquaintances different sizes but never double before my eyes.
Actually I guess I just didn’t pay close enough attention…thinking back I do remember times where I recall thinking they were bigger than I remembered but with more important taks before me that line of thought never got airtime in my head.
Ladies…if you notice me staring at your breasts please don’t take offense as I will be conducting a research study for the next 50 years or so on breast enlargement due to sexual arousal ;).
If a woman starts to touch you more when your talking (like gently touching your arm etc,) that could mean she’s aroused.