You’ll want to look and see what ports are currently in use on the network. The simplest way is to use a network analyzer such as WireShark or, ideally OmniPeek (its ‘monitor’ mode is perfect for this since it doesn’t capture all packets) to see what’s running.
You can also look at your servers/workstations to see what’s running as well (netstat -an|more at a Windows command prompt). Usually the higher the port number, the less likely it is to be in use.
Here’s the official port number listing by IANA that’d be good to check as well: