Many non-verbal cues are culture/ethnic specific and signal different things than a "standard predator-o-meter" would be reliable for. Eye contact, personal space (distance between two people), and touching are three variables which come to mind. I've traveled throughout Europe and the Middle East, and many times have gone unnecessarily into "alert mode" because of these three variables.

Two times I thwarted pickpockets with aggressive and dangerous posturing on my part. One of those times it was a gypsy team in Amsterdam and I picked up a weapon to back them off. Never been mugged (thankfully).

YMMV, but my plan is to use the cues I'm used to when deciding about going on alert, but be conservative with counter-punching. The stranger who stared into my eyes, put his arm around my shoulders, and pulled me close to him in Damascus was trying to sell me a carpet and not mug me. The guy who tried that in New Orleans evidently had something else in mind.