Mail the High Priest, High Priestess, and Council members. Get to know them as they might be your coven members. Ask questions about the coven, this will help you feel out what is the right fit for you.
You can also discuss the environment itself and what sorts of topics come up. If you have particular interests you can ask if there are any posts on it in the forums or if anyone is teaching that particular topic. If the coven is a smaller coven, you can start a discussion on why and how/if they expect to grow. If they are larger, asking questions about how much one on one time you might expect to get can be helpful. You can also ask coven members if you can discuss their coven with them.
Additionally, it's always helpful to check out the priest and priestess to see how active they are online and within their coven. (If a coven doesn't have active leadership, it changes the dynamic going on.)