Easy-peasy. Look for foliation on the gneiss. Granite is more massive.

A gneiss will have foliation to it (1st pic) – parallel lines that are the result of a pressure/stress regime that forces the minerals to run perpendicular to the stress field. Granites that have not been subjected to a stress field will be massive and no indication of foliation (2nd pic).

This can get tricky when a granite turns into a gneiss – they will look similar, but you have to look for the foliation.