How do you tell the difference between the 2?

Ok my husbands mother gave us her wedding set from her marriage to his dad. It's a very unique set, one that no one else will ever have "she designed the set herself" I am just curious though how can you tell the difference between a diamond and a cubic zirconia? I am very aware that they paid quite a bit for the set to have it made, and I am very thankful to her for giving us the set, but I want to have it appraised, and don't want any surprises while doing so "being told it's not real, etc." because I think that would push his mom over the edge with his dad. "She designed the ring, but he had it made" So that is the question I was wondering about how do you tell the difference between the 2?