They're usually the guys with testicular atrophy. Just ask 'em I'm sure they won't mind letting you have a grab.
But in all seriousness. What fucking business is it of yours and why do you care? So you can justify lifting less weight than them or looking smaller than them? Do you go to the gym to be bigger than everyone? To impress the ladies? Or to challenge yourself to be the best/biggest/strongest you can within your personal guidelines (i.e. drug usage, total invested time etc).
The only people who should care if someone is on crank are those in amateur sport where steroids are not supposed to be used and give a SERIOUSLY unfair advantage to an athlete who uses. Even in cases like these, unless you can prove it via testing, GET OVER IT.