How to know if you are a good communicator 

You are a good communicator if you first are prepared, you can not tell something because the audience is not going to take seriously your speech. After all you have to be aware of your body language, that tells more than thousand words, you can transmite the idea more easily.

If you get, no matter how, the attention of the audience you can do whatever you want with them, it is more easy to transmite the idea, but also have to be aware of losing it.

Finally if you are once of those that get to the point, you are a good communicator.