How to tell if a book is a 1st edition?

I have seven Nancy Drew books that I think are first edition. They're hardcover (missing the dustjackets) and the only date listed inside says "1935" or "1937". Someone told me that if they weren't first editions, there'd be another date (the reprint date) next to the original???? is this true? how can i figure out if they really are first editions?