Depends on the type. Can usually see the pregnant females np. When lights are out throw in some food, wait a few minutes and get a flashllght and look down at rock bottoms. If you have a fuge, put the light on and look.
Something you should do is add some new pods, or rocks, or macro, or ? that you know has some every year or so, so the get new dna and they don't stop reproducing because of inbreeding.