The Washington Times did an article about how meaningless PG-13 can be. We've obviously noticed it as well. Some movies with that rating really should be "R," while others could just as well have been PG. We refer to "hard" and "soft" examples.
When I'm trying to make a decision, I'll look to Screenit. It is the most comprehensive, detailed and explicit, going as far as actually counting and detailing cusswords and problematic imagery. The drawback is that you have to pay to see reviews of brand-new movies when they come out; otherwise, you have to wait around three days after they hit the theaters.
I also use Christian Spotlight, but with a grain of salt. Their reviewers are just people who get into the movies then write. They aren't given reviewer passes, so reviews won't be out until the movies have been in the theaters. Also, reviewers are of widely varying quality. I've thought some reviewers absolutely clueless. Yet they always give the movies artistic rating and moral ratings, and usually comment on the movie's suitability for kids. Plus, they print positive, negative, and neutral comments from folks who have gone, giving their ages. Sometimes those are more useful than the reviews.
Besides reading major reviews such as Ebert (good writer, usually fun to read even if I don't agree — but morally largely unconcerned) and trying to hear or see if Michael Medved reviews a movie, those are my main resources.
What do you use?