It takes awhile, but eventually you begin to kinda get a sense of a guy's motives

I've had guys smile at me, some seem to be just being polite, some seem to be somewhat interested and others were obviously kinda creepy
The guys that wolf whistled and followed me around in their car were creepy, the guy that followed me around with his huge lifted 4x4 pickup truck on a few occasions was creepy, the guy that followed me around the grocery store saying rude things and had to be restrained by store employees was definitely creepy, the guy that was staring in at me through my apt. window and then walked right in my door a few moments later was not only creepy but also scared the hell out of me
It is important to be aware of your surroundings but I try not to read too much into things unless it is necessary, otherwise I'd be a totally paranoid basket case