I just went through this. It was easy in my case. I only had one plug that had "higher than normal" resistance. #3 was the problem in my case. It could be any of them. There are some other things to check too. Here is that thread:
http://www.peachparts.com/shopforum/showthread.php?t=259616&highlight=glow+plug+light
Follow the tutorial over at DG. The system is pretty easy to troubleshoot.