The answer to this is found in Chapter 10. In that chapter, Pip is at the Jolly Bargemen when a stranger comes in and wants to buy people a drink. It soon becomes clear to the reader that the stranger is the convict that Pip had helped. He later gives Pip a shilling and the bank notes.
When he first comes in, we can already kind of see that something is going on. The man is looking funny at Pip, as if he is trying to tell Pip something. Then he keeps doing things like rubbing his leg very significantly. This (we know as readers) is a way of referring to the shackles that had been on his leg. But the thing that really tips us off is the fact that the man stirs his drink with the file that Pip took from Joe and gave to the convict.
All of this tells us that this man (who gives Pip the money) is really the convict.