Testing for copper?

[Doormatt]

From what I've read (only a few places) it's very difficult to test for copper levels.

My FW tank that I plan on moving to saltwater was bought used, and supposedly only used for a few months before I bought it. The silicone shows no discoloration from copper use, and the tank has been copper free (in my hands) for the last two years. Should I be worried about possible copper contamination, or is there an easy way to test?

Thanks!