Now I want to say something to comfort the victims of fallen preachers: it wasn't your fault. Romans 2 heavily condemns people who judge; the reason you didn't spot the phony preacher is that you're not committing the sins that the fake was committing.
Wikipedia actually has an article on evangelical preacher scandals, and two prominent names do not appear. Billy Graham's "Great Los Angeles Crusade" launched him into fame, and 25 years later, his entire staff was still with him. His standards of honesty are so effective that they have been adopted by other Christian organizations. Yet Graham regularly appeared on Jim Bakker's show, and he had Katherine Kuhlman with him in some of his crusades.
Why couldn't Billy Graham, a man renown for his integrity, not spot these two fakes? Because he himself is Godly, and he did not recognize the sin in their conduct. You weren't deceived because you were stupid; you were deceived because you don't recognize ungodliness. You didn't recognize ungodliness because you aren't ungodly yourself.