Related Video: How To Flirt Like A Pro

1. IT'S A CHEMICAL ATTRACTION
Pheromones, nature's sniffable love potion #9, provide us with important information about compatibility. Though aftershave or cologne can mask a man's pheromones, a recent study showed that people tend to pick perfumes similar to their body's natural odor.

"Perfumes enhance one's own scent and, thus, entice members of the opposite sex," says biological psychologist Dr. Nick Neave of England's Northumbria University. So if you like a man's cologne, it could be a sign that you're compatible—at least biologically.

2. MR. BIG STUFF
According to Michael Marinoff, a bartender at New York's Maritime Hotel, the modern Casanova wears many disguises. "Players have become much more subtle," he says. "It's all about fusion."

If a guy looks like he's trying to merge many styles (e.g. a Brooks Brothers shirt with the sleeves ripped off), he could be attempting to multiply his options. If the conversation turns intimate too quickly, run for cover. The quickest way of detecting a player? "He buys you a shot of Southern Comfort and lime or a Red Bull and vodka," the man-about-town's drinks of the moment.

3. IF IT WALKS LIKE A DUCK ...
We can absorb as many as 10,000 nonverbal cues in less than a minute, says body language expert Patti Wood, and can form impressions in as little as one-fortieth of a second with an accuracy rate of up to 80 percent. So, check him out.

An overly broad stance could mean he's domineering. Are his feet pointing toward you? As Wood says, "Where the feet go, the heart follows." The timing of a smile is revealing, too. If he smiles before he speaks, he feels good about you; if his grin follows his speech, he could be faking it.

4. PERP TALK
For the women of the 78th Police Precinct of New York City, spotting a shady character comes down to "Gut feeling, with both men and perps." Signals of sketchiness include lack of eye contact and fidgeting.

If his eyes roam over your body instead of resting on your face, you can probably guess his motives. And if he changes his gait or begins to perspire profusely upon spotting a cop, watch out—"Oh, do they sweat."

5. CARNAL FOREKNOWLEDGE
"In 10 seconds," says Sheldon Burman, director and founder of Chicago's Male Sexual Dysfunction Clinic, "I can tell you his sexual orientation, I can tell you if he's an enthusiastic sexual participant, I can tell you if he's seething with rage."

Short of scheduling an appointment with Dr. Burman, how can amateurs judge a potential bedmate? Warning signals include a sallow complexion, labored breathing, shaky hands, poor coordination, or the general appearance of poor health. According to Burman, "if they look unwell, they're probably not functioning sexually."