By examining the religion, it's beliefs, if there are any contradictions or inconsitantcies, how it was formed, who formed it, where it was formed etc.
Taking Scientology for example. It was formed by a man who originally stated that they best way to make money was to form a religion. It involes giving a lot of money to the religion. It is a very recent religion formed by a (poor) sci fi writer that seems to have some underlying (poor) sci fi themes to it. The way it operates, the people it targets and it's "belief" systems would all logically suggest it is a complete pile of rubbish. Logically I would have to say it doesn't offer the truth. I, of course, could be wrong, but I don't think I am.
Flying Spaghetti Monstorism also has similar flaws, though probably rates a touch higher in the believability stakes...
You can take the same methods and examine the other major religions and probably come to similar conclusions.