How will SETI confirm a finding?

SETI (Search for Extraterrestrial Intelligence) has been scanning the heavens for decades and so far have found no hits. Here's the question: if they ever do receive an irrefutably "intelligent" signal, how can they be sure it wasn't either an error (i.e., a signal from earth that bounced back from some object in space) or a deliberate hoax? The latter is especially likely, since lots of clever people would love to make SETI the butt of a fabulous joke? Unless I'm missing something, the money spent on SETI is just going down the toilet! John Gamel, MD