>The thing is the actual printing command is issued by another application I have no access to. So I thought that maybe I had some low level API function or something that could "listen" at a printer port and capture data being sent out to it by this other application. Thanks for your help, Pablo.

It's a PortMon from www.sysinternals.com :
"Portmon is an application that lets you monitor serial and parallel activity on your local system, or any computer on the network that you can reach via TCP/IP. It is the most powerful tool available for tracking down port-related configuration problems and analyzing application port usage."

Alexander