The gamecubes do not have a warranty seal to show you if it has been tampered with.  There are some ways however to tell.  Before buying be sure to get pictures that are detailed and ask many questions.

If the open button is somewhat sideways and not flat this is an indicator.


Another way is that if there are pieces missing around the two case halves.  This shows that someone pried there and didn't know what they were doing and cracked a piece off.