A way of thinking about (and hopefully remembering this) is that with pronouns an apostrophe is always a contraction, and the possessive pronoun doesn't have one. So, its, his, hers, yours, theirs, whose (possessive) and it's, he's, she's, you're, they're, who's (contraction with the verb 'to be'). Obviously it's easier when the root word changes between the two (eg she's uses nominative case and hers derives from the accusative), or the verb is plural.