I wish it were easy to identify a rumor or urban legend by its content, but I don't think you can; at least I can't think of any simple rules that I think are reliable.

I wish you could say "err on the side of caution" but even that is ambiguous. People can differ in their perception of which side is the cautions side. For one person, it might mean, if you hear a rumor that X is dangerous, assume it IS dangerous until you see proof that it isn't. For another, it might mean, assume it is NOT dangerous until you see proof it IS.