I don't believe that it was a philosopher that coined this phrase. Think of a farmer in the middle ages. Say this farmer was told a story of a magical Chinese powder that can topple walls ten feet thick and light a forest into a blaze. he was probably somewhat skeptical, wanting visual evidence. He might have said to whoever told him this "I'll believe it when I see it." One of the few comparisons that I can see in today's standards would be someone telling you that the government was now equipping the army with hand-held laser rifles. How would you feel?
Hang on. Seeing something is not the answer to scepticism. It is the reality of what we are seeing that scepticism questions. Someone who says that seeing is believing is sceptical to some degree, but they are not a sceptic in a philosophical sense.