What was Big Willy Shakes going for?
Shakespeare's saying comes from an old proverb, but it's as up-front as it gets. It has to be; it's about the truth after all. Lancelot is saying that the truth will come to light, or, that his dad (Gobbo) will learn that it is him (Lancelot) eventually.
He's joking around with his blind dad that his son (Lancelot, himself) is dead. Too bad he's the only one laughing. We're not surprised when his dad doesn't know whether the man he's talking to is his son or not. He's blind, and then he's just been taken for a ride by some guy claiming to be his son. Poor guy.