I live in the suburbs a metro area of about 6 million and work in the city, so if I were to start focusing on blacks, hispanics, guys with tattoos, etc., I'd be exhausted all the time.
I try to stay on my toes, but these types of "obvious" cues aren't of much help in most situations. Having said that, when I'm in my almost all white neighborhood, I have to admit that my ears perk up a bit when I see someone who, by virtue of their appearance, looks to be an "outsider." Still, that's not of much use to me. Around here you run into humans of every imaginable description.
It may not be scientific, but I'll admit that I go mostly on my gut feelings. If something doesn't feel right, I try to make a dignified exit from the area. There are a couple of things that always set off my alarms, though:
First, drunks and their friends; I've seen enough situations where booze (or drugs) has caused people to do things that endanger themselves or others to know that the danger level increases substantially when people are intoxicated. Aggression without apparent provocation happens all the time. Drunks who are on their own, wandering through the neighborhood, cause me the most concern.
Second, groups of young-ish males hanging out, trying too hard to look casual.
Third, anyone wearing gang-related apparel.
I don't get too weirded out by people who look bizarre. Others may disagree, but nose rings, piercings, and vast landscapes of tattoos don't bother me. I usually engage them with a "Hey, how's it going," or some other small talk and almost without exception get a positive response. Maybe it's because I'm such a square-looking guy (50-something with short graying hair) and they're not expecting it.
So, what I'd urge--apart from steering clear from badasses, like those who identify themselves with gang apparel--is to trust your gut. If you've got a bad feeling, it's worth heeding. Your brain, operating on a subconscious level, is capable of accurately processing much more information than your conscious brain, trying to figure out if A+B+G=danger. So, listen to your hunches.