The rating on a movie can only give the vaguest idea of what is really in it or whether it's appropriate to show to your family. One movie might be completely clean yet earn a PG-13 because it deals with a terminal illness. Another movie with the same rating could be peppered with vulgar language, sexual references, and violence.
Different parents are comfortable with different things, and some families have tools such as the TVGuardian (which utilizes the closed caption feed to silence and remove profanity and blasphemy), so I think it's more helpful to be able to find out precisely what is in each movie. Even with the new extended rating boxes (which indicate that the rating is for 'langauge and innuendo' or 'thematic elements') it's still difficult to really know what is in the film. For this reason, I like to utilize movie content reviewing sites.
I used to use ScreenIt, but their site is now harder to get through because they have two levels of reviews--the simple ones are free, but you have to register (and pay) to get the more elaborate ones. As you navigate through the site there are frequent ads for the paid version (which is ad-free) and it's somewhat annoying. If you don't mind the ads though, they have very detailed (blow-by-blow) information about the movie content.
My sister recommends Kids-In-Mind, which I have only tried once, but it was like ScreenIt in that it has extremely detailed information about each category (sex/nudity, violence/gore, profanity, and substance use). For example, it would say "a man and a woman kiss, and he asks her to come up to his apartment. Nothing is shown, but afterwards we see them laying next to each other in a bed." Under the profanity it is very specific. eg: "3 scatalogical terms, 2 sexual references, 1 anatomical term, 5 mild obsenities, name calling, and 5 religious exclamations." The only downside is that they seem to stick to the bigger movies, so if you're looking for something that was released straight to DVD, or that's on the older side, you may not be able to find it here.
I also like CommonSenseMedia because it has a wide variety of movies and doesn't require registration. It has brief and simple reviews, but viewers can also contribute reviews and I find the multiple perspectives helpful. Each rater (both the official site rater and also each viewer rater) gives a rating for what age they feel the movie is appropriate for: it's much more specific than just "PG" vs "PG-13" because you can rate it for any age 2-18. (If you want to leave reviews, you will have to register, but registration is free.)
So that's what works for me. Works For Me Wednesday has now moved to We are THAT Family, so now you can run on over there to see what works for other people!