when no one else is around, drop a piece of paper on the floor in front of him. bend over and pick it up with your backside to him...slowly. after you pick it up, turn around and see if he's doing it again. there's your answer.
Nothing has to be revealed... I go to work on the train. There is this lovely asian woman mid 30s, always dressed formally, never revealing and quite attractive. When she sits down she always fixes her hair at the back in the same way... And thats it... Unfortunately it's my stop next so I have to tuck it under my belt and move quickly...
revealing isn't necessary to produce desire, but she specifically wants to know if he's getting a boner because he's looking at her.
I was referring the mention of "showing a little cleavage". I was making the point that it's probably not the cleavage but the woman that is provoking this reaction.
A few years ago, I was not normally into white women, but I had a part time job working for a furniture store, and this young demur white woman comes into the store. She's was the Chealsy Clinton type, but with a bubble butt and hips that would put the most voluptuous black woman to shame. Nonetheless, I was sitting down doing something while looking at her, and that was the very first time that I ever got a hardon so intense for a total stranger, that I couldn't stand up for a good ten minutes.