How do detect a port scan?
 

I was wondering how someone can tell if you port scan them through a linux box?

I am running a linux box and I'm considering setting it up as a router, but I'd like to have complete control over detecting port scans and everything, how would someone do this?

Go to sourceforge and download the package 'Snort'. It is a really cool intrusion detection system as well as other functions it provides. It has paranoid port scan detection and can detect all sorts of other exploit attempts.