I rarely notice it. Oddly, I notice it more often if I’ve had a few drinks. Otherwise, I won’t know unless someone comes up and tells me “hey, that girl was checking you out.”
I also tend to disbelieve it when a male friend tells me a girl has checked me out; I tend to only believe it when a female friend tells me, or someone in the party of the girl who’s checked me out lets me know.
Are you sure it’s not just the alcohol? 
Yeah, I’m not sure I’d believe a male friend either.
I think I notice a woman looking at me about once every six months. What that means is not for me to say.
I’d like to say, for the record, that: No, I do not. And I am sorry. 
Why? Well…I recently went without sleep for 24 hours so I could get a chance to see antique core memory planes at a museum. I think that sums up the issue concisely enough.
I’m pretty oblivious.
Clue-by-fours tremble at the mention of my proud Westrogothic forehead. Which, I’m sure, is a comforting fact for my wife.
Unless she makes it really obvious then no I don’t notice. Subtle looks or cues from a woman are apparently wasted on me.
There is this really pretty young woman… girl really who is always smiling at me and going out of her way to talk to me at the local grocery store ( she works there ). The problem? She can’t be more than 19. I don’t know if she has daddy issues or what but I’m not going to find out. Even I’m not that desperate. Yet.
Could she be some sort of medical professional that you see for regular checkups?
The knee-jerk response to the OP that came to mind for me was "Well, duh. They’re continuously checking me out. I’m just that hot.
More seriously, I’ve been told that I’m simultaneously fairly dense and confusingly hard to read at times. On one occasion, a female friend of mine, upon realizing that I failed to even ask for a phone number from a cute female Marine (yeah, they exist!) I had been chatting with for about 20 minutes, took it upon herself to track the girl down and get the digits for me.
I do remember this one time at a previous base I was at where this cute redheaded Army girl at the bus stop scoped me out like I was a juicy steak and she had missed lunch. That was nice. Caused me to walk past, stop, consider, then spin around and start a conversation.
Turned out she was fresh out of Basic (yay “Basic Goggles”, not to be confused with BCGs), AND engaged to be married. But yeah, I noticed that time.
Basic Goggles don’t hold a candle to the power of Deployment Goggles. But anyway, no, I don’t notice. I can’t understand it, either. I’m pretty hot. I mean, I’m a semi-muscular 23 yr old. I dress well. I’m never short on conversation. So either I have my self image totally backwards or I don’t notice being checked out.
Yeah, I’ve thought about that angle as well… 
It has frankly turned out to be pretty accurate; I’d say my success rate (getting a number or being approached or whatever) is nearly 100% for those times when I think I’ve been checked out. Of coure, I’m also more likely to reciprocate the flirting if I’d had a few, so there’s a chance that I’m accidentally wooing women who weren’t initially checking me out.
Oh, no question, I’ve a lifetime achievement award for oblivious.
Rarely. Hell, I rarely even know if a woman is interested, let alone know if she is subtly checking me out.
I notice very occasionally, which is to say that I have no idea if it only happens occasionally or if I only notice occasionally. I would guess about 50% of each.
I only notice at concerts, which is bad because social interactions in a place you can’t be understood at is even worse than a place you can.
Plus, I’m probably not checked out at places other than concerts. I changed pretty smoothly from a gangly-looking college boy to an average balding 30-something guy. But when it’s dark you can only tell that I’m not gangly looking
Another oblivious person here. The only time I did realize it was pretty obvious, and she later told me that I was a dumb idiot for ignoring her. If I happen to have some female friends around, they help me by pointing it out.
If I think about it, I feel bad thinking about what I have possibly missed!
I have occasionally been aware of being checked out by a lovely lass…
…after my wife pointed out to me that she noticed it.
Generally, yes I can. There may have been some who were checking me out so shyly that I did not notice; but I’ve never been embarassed by hitting on a woman who I mistakenly thought was checking me out.
Women are about as hard to read as large print books.
Men are more like billboards.
I can always tell.
Unfortunately, it never happens…
Ah, now… I don’t think it’s fair that women can be mad at men for being dense. After all, if you’re actively trying to hide your interest, I’d say the men in question were acting just fine 
ahem… in other news, I am also quite oblivious. I’ve had a girl ask me to bed once, and I assumed she meant sleep.
I’ll let myself out now…
I’m generally oblivious. When we were regularly out and about my wife used to tell me someone, male or female, was checking me out. I was clueless. One time I was casually bopping with a lady friend while watching a band and she told me this girl turned around and absolutely beamed at me. I missed the whole thing. Of course I didn’t follow up with anything because I’m too tongue-tied most of the time. Nowadays sometimes walking down the sidewalk I’ll look over at a woman and she’ll look away or we’ll make a little eye contact so I’ll make the assumption.