You can use the list of the controllers returned by nltest (in the answer above - How to determine existing AD sites) or the alternative is to run the DNS query using nslookup command.
nslookup -type=SRV _ldap._tcp.dc._msdcs.<your domain>
Example:
nslookup -type=SRV _ldap._tcp.dc._msdcs.domain1.mycompany.org
This DNS lookup is what the IOPLEX Jespa library is doing internally and should return the same records as nltest command.
Then for each name, you can run the following on the server.
nltest /server:<hostname> /query
Example:
nltest /server:dc01.domain1.mycompany.org /query nltest /server:dc02.domain1.mycompany.org /query nltest /server:dc03.domain1.mycompany.org /query
The above command clearly identifies which Domain Controllers are actually reachable from the server.
You can then correlate this information to the site names and choose the site to be used in "AD Site" parameter in EasySSO config GUI.