my best guy friends and even my exes have told me all guys check out a chicks ass/body. it's just a natural thing for them. even when you don't think they're looking at you they're looking. which has kind of made me feel self-conscious lol.
I hardly ever check out a girl when I think she is looking and I still get caught a lot especially before. like there was a time in grocery store where a girl was at least 15 feet away turned and she turned around and looked directly at me. I personally am wicked obivous (not on purpose) as soon as she goes to turn or look I emidiately look away and if I can't look away fast enough or I no having warning I get into an awkward staring contest and always look away first. I never manage to play it off like everyone suggest
I find the best way is to check her out when she know you are looking, let's her know you're interested. That way you don't get "caught" cause she know full well you're checking her out.
Its kind of like a habit after a while you don't even relize your doing it. And if I do get caught I useualy look away but sometimes I may smile if I'm feeling curagous lol.
You can't honestly say you don't already know the answer to this question, of course guys do, only idiots get caught tho.