It might be hard to hear. Clapping is good enough to hear flutter echo, which is a nice and easy place to start. When flutter echo is bad, you'll hear a "zing" or "sproing" sort of tail to the sound. If you can't make that out in the theater, that's fine and good. Try clapping in a stairwell or parking deck. I'm of the opinion that flutter echo is just the beginning and the only thing you'll be able to identify by clapping, but I suspect others disagree (Nightlord). He may have a way for you to hear more spectral details of the decay, but I don't have a lot of faith in that.
Another easy way for you to begin to hear differences and the effect of the absorption is to stand right up next to it; one ear to it and the other out into the room. Get a simple recording playing - especially one without a lot of processing or ambiance added, or just get a conversation partner - and listen carefully, paying attention to the differences betwee what each ear hears. Stand next to untreated wall, and then move in front of the treatments.
Being able to hear subtle changes takes practice. I'm not good at it, but I'm working on it.