How do you know if someone is a great Communicator?

When he have the ability to pay your attention and his ability to convince you

A big part of communicating well is responding appropriately to what someone else communicates. This requires knowing how to actively listen and making every effort to understand what they’re telling you.

Have verbal and written communication with that person. Try to know does that person likes to read what type of books he/she reads then does he likes to write also try to read his/her writings. That will give you a brief idea either that person  is a good communicator or not