Jesse from Bent-Tronics has posted a tutorial on how to make a DIY MIDI tester:
A quick and dirty (and cheap) MIDI tester. Sometimes you just need to know if a MIDI controller is outputting anything. Even many high-end MIDI modules/boxes will have a “MIDI Activity” light, just to let you know something is being transmitted/received. That’s what I have made in this video.
Using a sacrificial MIDI cable, an LED, and a 220 Ohm resistor, you too can make this tester/activity light, probably in less than 10 minutes.
More details at Bent-Tronics homepage.
Check out the video after the break.