Pretty much every testing method (within financial reason) is destructive in a small way , since they require that a sample be taken of the material .
A simple destructive test is to remove a very small sliver ( from somewhere on the bottom rim areas , such as inside of the rim or from the base ) with a razor blade . Place the sample over a lighter (hold it with forceps or a hemostat) ... melts quickly = pewter .
Another less painful way is to take it to a local jeweler (call them first & maybe shop around) and have them test it while-u-wait .
It's also possible to do a specific gravity test on your item , but it requires a really good scale , submersion in water & accurate math skills .
I think the second option is a better one for you .