Diamonds don't hold a fog with a breath test. Just HhhhAAAAA on it, and see what happens. If the condensation sticks for a long time, you've got a stone or CZ, if it pretty much disappears quickly, you can be reasonably assured it's diamond.
Because this test depends on relative humidity and temp, it's best to do it while breathing on a stone of known "fakeness" or a piece of glass (like your glasses). If you've never done it with a real diamond, it helps to do that, too, so you know what you're looking for.
Cheers,
Skippy