How to date a US cover

Not a dumb question at all. Unfortunately, since you don't have the contents, it gets much harder.

If there are no other markings on the cover, then:
-- sometimes IDing the stamp can give you an approximate year range (or at the very least, an earliest possible year)
-- sometimes info on the history of the addressee helps

In special cases, the postmark style and post office location can also provide approximate year range, but that requires considerable research and information is limited. US postmark & post office info is nowhere as comprehensively documented as, for example, Great Britain or France.