Ways to tell the difference of plastic and metal?

Sup, I'm a little bit of what you would call, a builder, and I'm making something that requires a small metal piece. The only thing is that it cannot be plastic. I found a piece thats shiny and round but its not even a 1/2 in diameter. and no bigger then a 1/4 inch thick with a whole in the middle. I need to know if its metal or plastic and the only way I can think of testing something so small is to burn it and see if it smells like plastic. Are there any other ways besides burning it to found out what its made of? Thanks in advance. -Max