How can you tell the difference between a zirconia and a diamond?

My fiance and I bought a white gold engagement ring with a heart shaped zirconia. It is really cute and didn't cost much (80 quid). Some high maintenance women wouldn't even look at this ring. But it means much more to me and we don't have much money and I also think it's such a waste to spend a month salary on a ring. It doesn't look cheap at all, it's very delicate. I mean how can a normal person tell the difference between a diamond and a zirconia anyway (unless everyone you know is a jeweler)???