It all depends on what the other drop-down in Gmail's Forwarding Settings was selected:
Gmail Forwarding Settings http://geekcubed.org/random/gmail_fwding.png
The options are fairly self-explanatory. If it was configured to
Delete Google Mails's copy then there is not going to be any record of what was forwarded. If one of the other settings were selected, then the mail would still be in the appropriate folder (Inbox or Archive).
Even if the delete option wasn't selected, you have to suspect that the culprit may have accessed your account again and just manually deleted mail.
Hopefully this will be a lesson - never give out your username and password even to people you know.
You are going to have to assume that [they] got hold of everything. Every account that points to that email address (e-commerce sites, banking, any sort of membership) should be assumed to be compromised, and will need to be checked and secured. They could quite easily have recovered the password to those accounts via your email and then deleted all trace