Excellent question - Edit to correct
- most places can only adjust 1 stage because those are used on older cars and trucks but any place can test them. I saw this DIY pop tester,
Make sure that you are shielded from the diesel spray - it comes out at a very high pressure and can get injected into your skin and give you blood poisioning.
IMO, the other side of the problem is that you need hardened shims from bosch to adjust the pop pressure. A homemade tester is fine for checking the spray pattern but you can't adjust it without shimming the springs.
There is a place called diesel fuel injection service that knows VWs well, 503-285-1213 is their number, they are located in washington state and they can do the adjustment.
From what I understand, the nozzles are what can go bad, the injector bodies are pretty reliable as long as they are kept clean. Pic below from http://www.myturbodiesel.com/1000q_how_to/multi/TDInozzleFAQ.htm
to help show the 2 springs.