Working my way through the Lion Server Advanced Administration documentation.
I'm at the section "DHCP server configuration for your server"
I have a Comcast Business Gateway and connected to it is an Airport Extreme.
How do I know which device is my DHCP server?
You will have to check the configuration of each. For the AirPort this is done via the AirPort Utility program, for the Comcast device it is probably via a webpage. However a quick-and-dirty method of checking is to have just one of them turned on at a time, and then connect a computer and see if it gets a DHCP provided address. If it does you know that particular router is running as a DHCP server.
Depending on your needs you could use the Mac server, the Comcast box, or the AirPort as your DHCP server. I don't know the details of the Comcast device but I have heard that some ISP provided devices are locked down such that you don't have the option to turn of DHCP, I can say the AirPort will let your turn its DHCP off.
Perfect, Steve! Thank you very much! :-)