I hate taking this to showdown. Without additional information we should be thinking not of what he could have, but what he could have that we could possibly beat. Then, when we figure out what the answer is we plug in "how often does he have that". The answer to that usually comes out "not very often at all". You mix in a bit of "how often do we pass if we're behind" (almost never) and compare the amount of times you're actually ahead to the amount of money you're going to have to put in the pot and instantly check-fold the flop.