Being able to let the dragons go is a dead giveaway only a Targaryen of Valyrian blood can get near and touch dragons. However he did not touch them, only their chains. They did however laugh at his joke. I'm' here to help. don't eat the help. To which Rhaegal laughs at hm. But yes the theory that Aerys II and Joanna had an affair and Tyrion was the result is very convincing.
RE: Do you know this from Game of Thrones? (spoiler)