This is one of the most tricky questions!
Chances are, when you use this plug-in, that you or your client will have a Network manager.
Ask him/her, they should know.
Unfortunately there are too many possible solutions.
A good help for us, during development, was the LDAP Browser.
It is a (Java) freeware tool and it will give you the base DN once you the url and credentials are correct.
You can find the LDAP Browser here.
Another cool application is JXBrowser, free as well and available here.
When you want to use an Eclipse (based) LDAP Browser you can read more about Apache Directory Studio™ here.