troubleshooting AFP beyond the GUI

We had an Xserve running both AFP and Windows filesharing. Both were working fine but then AFP starting failing to authenticate users. Windows filesharing continued to work. I tried to stop and restart AFP via Server Admin but that didn't solve the problem. I rebooted the server and everything worked fine. I'd like to know how to fix this in the future without rebooting the entire OS. I'm assuming I can kill AFP and restart it via command line but I don't know how to do this. What is the recommended method? Is there documentation that someone can provide that explains this sort of thing? How to troubleshoot OS X Server problems beyond the GUI? Thanks!