The ball check is easy. Look in the hole with the injector out. If the little ball is there, then odds are good that the PC is ok. I found 2 PC's in my 240D were missing the ball. When I pulled them and found them severly broken with huge chunks missing I was sure it was the cause of my rough idle. Installed new PC's, new heat shields on the injectors and fired it up! Guess what. Ran pretty much like it did before I did the work. Even with the busted PC's it ran good with great power for a 240D. So, my advice to you is to replace them if you see the ball missing or you have pieces banging around in there. Otherwise, keep looking for the real cause of your miss.

ps...... there are a lot of threads around that say without the PC ball your going to cut a hole in the top of your piston with the stream burning diesel fuel. After pulling the PC on my car I have decided that people who say that have never seen a PC in person. There is no way fuel is going to make it thru the tiny holes on the PC bottom to the piston crown. Im not saying a missing ball is ok, just dont believe you have cut your motor apart by running it without those balls.