It's not that in Iran (and Afghanistan for that matter) men are able to discern prostitutes through a complex calculus of the the amount of make-up they are wearing in relation to their coordinates on a street map. Coming-on to a woman, not to mention mistaking one for a prostitute, is high risk behavior in Iran. Men wouldn't risk jail, or worse, if it were such a hit and miss endeavor. Just ask the teens in Esfahan and Tehran who have come up with ingenious ways to let the other sex know they're interested. (Ever walk down a busy street and notice someone several yards behind you keeps blurting the same 7-digit number only to disappear once you've turned around to take a gander?)
Instead of these chananigans, prostitutes use a universally known symbol- they wear their chadors inside-out. I found this out because as a novice chador-wearer visiting Iran in 2005, I once accidently slipped mine on the wrong way. Thankfully, a roomate filled me in before we even stepped outside the dorms in Qom. After I knew what to look for, I saw it more than once.