Fortunately, there's a system.
You can generally tell a bad movie by the trailers and a couple reviews from reviewers you trust. To tell a good bad movie from the true dreck takes a little more research.
The keys to a bad movie worth watching are in the budget, the director (and to a lesser extent the writer), and the cast. In terms of the latter bigger is almost always better. A bad movie with a low budget often feels constrained and locked into its choices by its lack of cash. It also frequently features dreary visuals to go along with everything else bad about it. A bad movie with a big budget? That's an expansive flowing badness that you can really sink your teeth into. This is why Hollywood makes better bad movies than the Indie scene. Indies can be masterpieces, or really good movies, but without schlockey special effects and expensive set dressing they make dull and uninteresting bad movies. There's something glorious about seeing $120 million in special effects lavished on a script that could have been written by a four year old with shots that seem directed more by chance than by some grand artiste with vision or passion.
The cast of a bad movie is important for much the same reason. It's fun to watch rich famous people falter and fail. For example, In the movie Gigli there's enough schadenfreude to last you a week. Watching the Bennifer publicity machine recite its lines with the kind of confidence and self-awareness you expect at a middle-school class play is somewhat soothing. An added benefit to a famous cast is that sometimes a good actor can liven up a bad movie. Christopher Walken has, in fact, made a career out of this. You'll be watching some hackey scene where everyone's going through the paces like automatons when BAM, Walken walks in and adds a much needed note of creativity. Good actors can give good performances in bad movies. Famous actors are fun to watch fail. It's a win-win situation.
Perhaps the most important element, however, is the director. There are two types of directors who make good bad movies. They are the neophyte who has only worked in music videos and/or schlock previously, and the experienced auteur with established skill. The neophyte crap director will often be amusingly sincere in the way he directs, confidence unmatched by competence. This will result in something either campily fun (such as Anacondas: The Search for the Blood Orchid , a movie that cheerfully goes through its ridiculous paces with such only-in-film characters as a brilliant blonde geneticist with a thick southern accent who never actually says anything about genetics but is brilliant because we are told so three or four times during the film, in case we forget) or something so bad it's mind-boggling ( Mansquito!!!!!).
The director with a good track record, on the other hand, will usually produce a noble failure. Barry Levinson's Envy is a movie that seems like it really could work, but it doesn't, yet there are interesting ideas and visuals along the way so that you never get bored. Watching a film that could have been a good movie if just a few tweaks had been made is always interesting.
Of course even among the most promising bad movies there are always true duds. Uwe Boll is a crazy German who has decided to make a career out of creating English language films based on old video games few remember. That seems like a great formula for enjoyably terrible movies, but House of the Dead proves that sometimes a director can have all the right credentials and fail. The film is so atrocious it's not even worth mocking, because everyone involved seems both embarrassed and depressed to even be in the shit. Even the zombies look like they'd rather be doing something else. That's the danger with bad movies, that the viewer will slip from gleefully watching hubris to feeling sorry for a bunch of people caught in some moron's exploitation flick. If you follow the formula laid out above, however, you can mostly avoid those depressing examples. It's easy to feel bad for a no-name collecting SAG minimum to star in crap, it's much harder to feel bad for a smug Ben Affleck or a rising star like Jack Black.