We have two products that can help you with this objective:
Engineer's Toolset has port scanning tools that can help you identify systems that have port 21 open (which implies a working FTP Server).
Patch Manager has a network discovery tool that can be targeted to scan a specific port (i.e. 21) over a defined IP Address range, and then report on those systems with that port open. In addition, Patch Manager can inventory services, processes, and installed applications, which can be used to report on known FTP Servers that may be installed -- even if configured to listen on alternate ports.
Thanks! I spotted the Port Scanner in the Engineer's Toolset (sadly we don't have a licence for this), but didn't know about Patch Manager. Fortunately we have a licence for Patch Manager so i've started looking into this today, looks good!