In the new movie IT (2017), how did Bill Denbrough know at the end that he was not really talking to Georgie but to the clown? Was anything not correct that the fake Georgie said?
(This answer, obviously, contains spoilers for the movie.)
I don't think it's a matter of the fake Georgie saying anything wrong (and there doesn't seem to be a line in particular that Bill reacts to), but more of a representation of Bill coming to terms with Georgie's death.
He has seen Georgie's body being puppeted (?) by Pennywise in the earlier cellar scene, and therefore knows that the clown is capable of imitating his brother. When he shoots Georgie, it shows that he knows that his brother is dead and has accepted the fact that he will never see him again.
Of course, this is just my personal interpretation as someone who is a fan of the book and has seen the movie twice. There isn't really an set explanation for this - I guess it's up to the audience.
Hope this answered your question!