Anyone who looks like they 'know what they are doing' tends to get treated better by me, just because they are more predictable. I'm more convinced that they understand traffic and are used to what is going on, and aren't going to be making any erratic moves. Full road kit would let me know this, but anyone with the subtle touches of a regular cyclist will clue me in, like a crisply rolled pant leg, dirty messenger bag (clean ones don't count), dirty cycling gloves, clipless pedals, panniers, vests, helmet mirrors, etc.

If someone is wearing 100% street clothing with no slight modifications for cycling, and has a bike that looks way to clean for its own good, I assume that they aren't regular cyclists and are liable to do crazy or dangerous things. A generalization, I know, and not always true, but its all I've got when I have to formulate a plan of action based on a 1/2 second glance.

peace,
sam