Does Anyone Know How to Tell If A Car is Burning Oil?

My Scion has 75k on it, consumes a couple quarts every 3 weeks or so, and my mechanic tells me it's most likely burning oil and that the head gasket may be the problem. I've done some research on oil burning and have read that if your car is burning oil, you'd be able to see a cloud of smoke behind you as you drive. Is this the easiest way to tell. Could my car still be burning oil if there is no smoke coming out of exhaust? What other ways can I tell (without taking it to get checked out)? I'm just confused and maybe paranoid. Thanks