Q: How do I know when a Moderator is online?

A: In order to find which moderator is online, the first place to try looking is the Game Play forum (community>forums>game play). Find a topic that has been locked by a moderator recently. You can tell which topics have been locked by a moderator by looking for the green locks. 

Click on a green-lock topic and then scroll down to the very last post. It will probably be a post from the Moderator explaining why the topic has been locked. If the post is recent enough, that moderator is most likely online.

If you have moderators in your friends list, you are able to track their status there as well.