monitoring ports - Security

[jnewbie]

Howdy.
Well I have ipchains/Masq setup on my Linux box now but it will take a
while  until I master all the various ipchains rules.

Until I do, is there a simple way of me being able to monitor all
ports on my system for potential security breaches ?

I'll find out eventually how to protect myself better but for now all
I want to do is monitor suspicious activity not prevent it.