diamond question

[djugurba]

Any good, easy, fool-proof tests to distinguish between a diamond and an imposter gem?

(long story short... a couple at a bar where I was watching the Steelers a couple weeks ago attracted the attention of another man, who swore up and down that he knew the woman from somewhere. The guy (her fiance) got up to use the bathroom, and in his absence, everyone noticed the rock on her hand. While he was gone, the connection was made, that yes, the other guy knew her from her Playboy spread a couple months earlier. This guy wants to see her ring, and after a brief inspection insists that it's fake... he's a jewler, you see... to which she replies that of course it is not... a little uneasiness ensued when fiancee returned from the lav...)

So, I know that you can identify a few things by looking with a loupe (inclusions, etc), but without tools of the trade, is there any way to know for sure what you're looking at? I googled this and didn't find anything terribly helplful. So, naturally I thought the assembled mercedes owners would have the answer immediately.