What you don't see gets you.
If you have an IrDA aware printer, you can point your phone at it and you should see a light near the IR port light up or flash. If you have a Palm organizer w/ IR, point it at the phone. If the IR on the phone is in discovery mode, you should see the "Waiting for Sender" dialog box pop up on the Palm.
There is a program for the Palm called "IRMonitor" which measures IR emissions. You can get this off EuroCool or PilotZone. You run the program, point your Palm at where you think the IR beam should be coming from and if there are any emissions from that port, they will show up as a spike on the IrMonitor scrolling graph. I would highly recommend this program to anybody out there who plays with or works with IR devices.
You may also use a video camera to detect infrared light. But I couldn't check this yet.
Most of this section is taken from a posting to the Linux/IrDA list by "The Armadillo with the Mask".