...but you can't tell him much."
--American Military Joke
And like every joke, it has its roots in the truth. The first part does, anyhow; I don't care to test a Marine's sense of humor. United States Marines are amazingly easy to spot in a crowd. Other members of the U.S. military aren't quite as simple to identify, but you'd be surprised how easy it is once you start looking.
This node is intended to be a short guide to identifying people from a distance who may be U.S. military personnel, and is admittedly skewed towards identifying men (since the women in the military tend to retain their fashion sense even after training).
You've got the look
First of all, look at the individual. Everyone in the U.S. military is required to keep their hair cut within regulations (with the possible exception of Special Forces... but if you're dealing with them, you better already know most of this stuff). The haircut will not necessarily be the ubiquitous white wall or jarhead haircut, but some sort of high and tight or buzz cut. A male in the military will have short hair, and will rarely have any facial hair. The exception is a small mustache, trimmed so that it doesn't extend past the corners of the mouth.
Dress to Impress
The clothing worn off-duty by military personnel tends to be conservative, too. I call the following outfit the "official off-duty uniform" because it's such a dead giveaway. Look for khakis (or "dressy" blue jeans) worn with a collared golf shirt. The shirt is almost certain to be tucked in. Casual shoes will be brown or black (athletic shoes are less common) and shined if appropriate. Navy and Marine Corps personnel may have a white crew-neck T-shirt showing underneath the open button on their collar; Air Force and Army personnel will not.
Because of the etiquette surrounding the salute, many military members will carry a briefcase in their left hand rather than their right; this frees the right hand for saluting and holding doors. Likewise, a backpack will be worn over the left shoulder. Look for a watch worn on the left wrist (to indicate right-handedness) before you jump to conclusions, though. Men in the military will not have pierced ears, and unless they're out clubbing in something revealing, you probably won't see their tattoos.
Plays well with others
We hang out with the people we work with, by and large, so look for military members to travel in packs just as much as any other social group. Four guys with the same haircut and the aforementioned outfits hanging out with a girl who's clearly at ease? Odds are good they're military (or gay... or both). Listen for lots of acronyms in their speech. Like most Americans, military members tend to speak a little more loudly than absolutely necessary, and sometimes a little eavesdropping can tell you everything you need to know. The U.S. military is one of the most integrated workplaces in the country - look for an astonishing mix of races among the men and women. If the men are out with their wives, look for spouses from overseas, especially Japan, Korea, Germany, and Saudi Arabia (less common).
That "Je Ne Sais Hua"
That's all I can offer, but I will throw this last tip out: attitude. There's a difference in their gait, in the way they deal with waitresses and clerks (when's the last time you called the Starbucks barista "ma'am"?), and in their situational awareness. Whether you're using this guide to avoid them or pick them up, I hope the advice I've offered helps you find the military man you're looking for.
Disclaimer: Lots of other groups in America tend to wear their hair and clothing conservatively, often in the style of the military. For some groups, it's an effort to co-opt that air of uniformity and professionalism. For others, it's about looking "sharp". Nonetheless, if you follow this guide, you may hit some false positives among law enforcement and federal civil service (there's a big overlap between these groups and the military); emergency personnel (the uniformity is consistent with the discipline required to run one of these services); and the occasional non-military man, who just likes short haircuts and a sharp-dressed man as much as anyone else (and really, who doesn't?).
At the same time, you will get a lot of false negatives. Even in a group that values uniformity as much as the military, some people don't fit the mold. They wear their hair right on the edge of regs, they have a fake earring for the weekends, and they blend into normal society wherever they are. The only real way to know is to ask.