The movie Expendables may give you a good idea about who is really kicking ass and who is just acting. In fact, you can do your observation in any action flick: just pay attention to the body. if you don't see the full body of the guy while he is giving his best kick, he is probably not doing it.
Expendables is a good laboratory because it gives you a chance to see all those tough guys in the same movie, which makes it easier to compare. This is of course a very cheesy and very predictable movie, and as a matter of fact there is nothing much to predict at all. Yet, it is highly entertaining and it is really nice to see some of the old faces such as Stallone, Lundgren, and even Schwarzeneggar for 2 minutes. And no, Scharzeneggar is not fightnig. He is just a guest star.
So anyway, just pay attention to those moves next time you are watching an action movie. Now of course, it is all about make-believe after all, and maybe you just don't care if your guy is really a ass-kicker or just an actor. And after all, this is a movie and of course they should act, but somehow it feels better when yoı know that he is doing his stunts properly.