Dubious

dubious- doubtful; of unlikely authenticity

Dubious Derick is one of the most skeptical that I have ever met. He didn't believe anything that you told him, unless you have hard evidence. He probably wouldn't believe that it was raining outside if you came in with a wet umberella and soaking clothes. His favorite line was "Doubt it."

WTR - Dubious Derick was very doubtful.