Compare a contract against one you already trust — a fork against its original, or an implementation against the version you audited.
About contract diff
Fork a well-known contract, change three lines, and it will look entirely legitimate to anyone who does not read it closely. This compares two verified contracts and says whether they are the same code — with comments and whitespace ignored, since formatting alone is not a difference — and points at the first line where they diverge. The comparison that answers something is an unfamiliar contract against the original it claims to be, or a new implementation against the version already audited.