How does Daniel realize that Henry has been deceiving him?

Henry lacks serious ambition and is happy to defer most decisions to others, leading Daniel to suspect that Henry is not who he claims to be. One night while drunk, Daniel forces the truth out of Henry that he is an imposter, and kills him.

No offense to Jill D. but her answer is totally incorrect. Daniel becomes suspicious of Henry after reminiscing about his childhood town & makes an inside joke that anyone from his hometown would get, referenceing a local custom the "Peach Tree Dance" to which "Henry" has no reaction as if he doesn't know what Daniel is talking about. That's what tipped Daniel off that "Henry" was an imposter. It all plays out onscreen unambiguously