how can I tell if I have been hacked?

Answer 1

Assuming that you are running a good firewall program on your computer (not just the build in one with Windows), you should be able to go into the log of the firewall and see if any other computers have accessed yours.

Within a firewall, it is unlikely that you would be able to detect that you have been hacker, unless the hacker did some destructive to your computer that annouces his presence.