How can I identify who is using Sharing to access my Mac? How do identify who is using Sharing to access my Mac remotely?

I have a new 2013 27″ iMac using OS X 10.9.2
Why do you ask? What are the things you are noticing? If you haven't turned on your firewall, you should. (System Preferences>Security and Privacy). The likelihood of you being 'hacked' are very low regardless.