Working out is something you generally do purely of your own volition. It's not like the Army, or even a corporation, where you're expected to conform. So you can find out a lot about someone's personality from watching him work out.
Does he work hard at it? Does he break a sweat? There are those, mostly women, who don't even break a sweat on the treadmill.
Does he know what he's doing? Does he work his entire body, or just his upper body? (Any guy who works only his upper body is almost guaranteed to have an IQ below 110.)
Does he do exercises which utilize his entire body, or does he isolate the Hollywood muscles? (One guy can move well, the other generally can't; there's an IQ correlation here as well.)
Does a woman work her upper body as well, or just her legs and tummy and tush? If the latter, she is doing it purely for sex appeal, and has zero interest in athleticism.
Can he work out by himself or does he need workout buddies to keep him motivated? (Is he a peer pressure-type or does he actually have any will power?)
Does he prefer exercise classes? (Sheep alert! Or, possibly, voyeur alert.)
Does he vary his effort depending on whether someone else is watching? (If so, don't expect honesty in other regards.)
Does he/she spend a lot of money on his/her workout outfit? (There's an imperfect but generally inverse correlation between the expensiveness of the outfit and the seriousness of the workout.)
Is he more into lifting or cardio? One kind of guy wants to be more manly, the other wants to live forever, and each will probably express that dynamic in every other facet of his life. (Marathoners don't drive Ford 150 pickups, and power lifters don't drive Volvos.)
Is he the type who talks about his workout plan but always finds an excuse not to stick to it? (Don't expect him to stick to anything else either.)
How neurotic is he? If he misses an exercise, will it really bother him? (If so, don't expect him to be flexible in other regards.)
Does he use good form? Does he bridle if you suggest a change? (That's his whole personality right there.)
Is he willing to share whichever machine he's using? (A hog is a hog everywhere.)