I was wondering about this too, especially since it doesn't just go for TV and movies, it also goes to the internet. Here are some things I usually do.
For watching TV and movies, I always check the reviews on either Plugged In(although it tends to have spoilers) or Common Sense Media, both reviewer sites that just tell you what's in it and how much. I know at least one of them(Plugged In) is Christian.(the site is run by the guys at Focus on the Family)
I also base what's good and bad based on a scripture in the Bible(Philippians 4:8, the one that says "whatsoever is pure, whatsoever is lovely...") Basically, I ask myself, "Is there any virtue in this particular piece of content?(as in, is it wholesomely funny or do learn something from the characters, etc.?) If the answer is no, or the bad(lots of violence, swearing etc.) outweighs the good, then I tend to stay away.
That's my take. Hope this helps and God bless!