If I had to take a guess from my own subjective observations at:
1) Their career choice
2) How they type on an internet forum
3) How they talk on workout videos
I'd say most of them should be below 100, but there are outliers, such as Milos, Sean, Chick, Nasser, Jay, Bob Paris, Mentzer and other past iron agers.
Milos, Chick and Sean seem intelligent, regardless of their character. Guy's like Ronnie seem stupid and Kevin Levrone sounds like a retard. I hear Nasser has a masters degree in history, so I'm assuming he's smart. Jay seems well spoken, so he must have average to above average intelligence.
Someone should conduct a study.. 
Maybe MattC, as he seems like the perfect guy to do it.
Quote from Peter McGough, FLEX Magazine, on an editorial regarding Mike Mentzer's:
"Despite the many criticisms made about his training methods and philosophy, no one ever questioned Mike's intelligence and knowledge about the science of exercise. In fact, Mike was arguably the only bodybuilder ever who could have joined the prestigious Mensa Society." The cut-off for Mensa is a Stanford-Binet I.Q of 132 or higher. If Mentzer was the only bodybuilder who could have joined Mensa, it suggests that the average I.Q of pro bodybuilders is quite low indeed.
Personally, from what I've read about them, their educations, levels of intellectual curiosity, vocabulary and general logical reasoning skills evaluated from their training articles and such, I believe that pro bodybuilders average in the low normal range(I.Q 90-100) and the rest of them in the high normal range(100-110) and a small number, like Tom Prince, Chick, Shawn Ray and the like scoring in the above average(110-130). McGough is right that Mentzer is, arguably, the only pro who would be regarded as intellectually gifted(I.Q 130+). Such is life...
SUCKMYMUSCLE
