When the server is starting (via service), then you find in the error.log of the server a "listening on" line like the following.
[24/Feb/2017:15:29:44][64734.70000cee5000][-driver:nssock:0-] Notice: nssock:0: listening on [localhost]:8000
If some other service is running already on this port, you find a prior line in the error.log that listening failed.
