What I do is keep up with the ones that say "fresh" in the title. Basically, I rely on that tag and post count (10 or so is my preference) before jumping in and following it. By no means do I follow through on all of them, but it gives me some options.

If it is 50+, it is a little more questionable and looking at the post date becomes much more necessary to evaluate if it's worthwhile. 100+ typically means the RP is full and not really good for looking at, unless something in there explicitly says either there are folks always being accepted or if someone suggests it to you with the note that it would be able to take another member.

If it doesn't have the fresh tag but has under 20 posts, either the RP started somewhere else or just died off is what I see.

Basically, it's a matter of creating a system of how to check things out. I came up with my own, others have their own ways, and I'm sure you will get used to it as well :)