How to use effect versus affect even has a little test
From
Spelling and Grammar Rules You Should Know
WHO'S/WHOSEThis is one of those cases where it is important to remember that possessive pronouns never take apostrophes, even though possessive nouns do (see it's/its). "Who's" always and forever means only "who is," as in "Who's that guy with the droopy mustache?" or " who has," as in "Who's been eating my porridge?" "Whose" is the possessive form of "who" and is used as follows: "Whose dirty socks are these on the breakfast table?"
Who/Whom Usage
I found the who/whom very helpful just now as I read over the rule.