it's easy to tell what is free and what is not as the movie poster has a "prime" banner on it if it is part of prime. plus search feature has option to only show prime results.
you also get prime music, books, audio books, delivery with full membership. they work the same and offer a large chunk for free with others pay as you go. though the music part has been seriously nerfed to get folks to pay extra for music service. but still a lot to stream for free on it.