To find out about new movies, you can always listen to Plugged-In on SOS Radio every Friday at 6:40am PST.
Sometimes, though, your kids want to see an older movie that you haven't seen. When that happens, I go to the Internet Movie Database at IMDB.
You enter the movie into the search window. Click on it, then scroll down to the "parental advisory" section. They'll give you specifics on what's in the movie rather than just another rating.
I love that they do this as it allows parents with differing standards to decide for themselves what they would rate the movie (and if they'd find it appropriate for their child).