How can you find out if your computer has been hacked

Question

For some reason, I couldn't find any information on how I can tell if someone has hacked into my computer or not.
I have an iMac:
  Model Name:    iMac
  Model Identifier:    iMac8,1
  Processor Name:    Intel Core 2 Duo
  Processor Speed:    2.66 GHz
  Number Of Processors:    1
  Total Number Of Cores:    2
  L2 Cache:    6 MB
  Memory:    2 GB
  Bus Speed:    1.07 GHz
I was trying to save something on the computer and noticed that a co-worker's name was under the SHARED section in one of mu folders. He was in my computer! How can I find that information? His name was actually listed so I know he was on there. I even texted him then he stopped.
I have Verizon DSL and and older version of Norton Firewall (4).
He's my subordinate and we don't usually have problems. What do I do?

Answer 1

You probably made your system available to him by opening File Sharing. Open System Preferences - Sharing - File Sharing and look under Users. I you see it's open to him or everyone then highlight that and click the - symbol to remove them.
If you are using Norton then remove it following Norton's instructions. Their software tends to create more issues than it solves. OS X has a firewall built into it (System Preferences - Security - Firewall tab) so having a redundant firewall is unnecessary and a waste of system resources and a waste of money.