Building on what 4wilmar said...
Consider the kinds of loads you would expect in a weight room:
1. Most weight benches, people's feet, etc. impose somewhat heavy concentrated loads on the floor. I say somewhat because, in the grand scheme of things, the world record bench press (assisted - mucho controversy, web search for it if you're interested) is hovering around 1,000 lbs. or about a kip. The world record squat is, last time I read about it, something like 1,200 lbs, and the world record deadlift is also something like 1,200 lbs. You never know how strong some people are and there are some extremely strong high school football players and other who might frequent the gym. Or you might have pro wrestlers or others who are lifting this much weight. Still, it's "only" a kip. Thusly, unless the benches are bolted to the floor, design for concentrated loads in every worst possible configuration you can imagine. Example, someone is squatting with 1 kip. That's two 500-lb concentrated loads (one from each foot), right in the middle of the slab.
2. The really heavy concentrated loads don't come form the users, they come from the stored weight plates. A squat rack or power rack or whatever it's called can accomodate at least 2,000 lbs worth of 45-lb and less plates. Still, that's only giving you a couple of kips. There's also equipment loads from exercise machines (that sometimes are weight plate storage as well) and from exercise machines being moved in and out of the gym.
3. As 4wilmar says, there are repetitive impacts from dropping weights, so definitely requrire some impact absorbsion device like rubberized flooring, plywood (old school), etc.
4. Calling the equipment manufacturer is certainly acceptable and you can't really go wrong with their recommendations (unless they're in cahoots with the concrete plant..). Also, you can rationally interpret ASCE 7-02 section 4.2.1 "Required Live Loads" to be the maximum loads expected - since it's a weight room, design accordingly. ASCE 7-02, sect. 4.7.2 provides guidance on impacts from machinery.