How do the readers know that the bank notes came from the convict?

The strange at the pub man clearly knew something about Pip assisting the convict and wanted Pip to know that he did. At one point, the man stirs his drink with Joe's file -- the file Pip stole to give to the convict! As Joe and Pip depart, the stranger hands Pip a coin wrapped in paper. When they get home, Pip realizes that the paper is actually a two pound note.