I've spawned a close relative, the Orange Spotted Sunfish and the process is basically the same, except Orange Spots are much smaller and much less aggressive. Basically they need seasonal variation in temperature and day length to spawn. They should be allowed to cool down over the winter and day length should be shortened, preferably to coincide with natural light. Then in the spring raise the temps and lengthen the days to stimulate spawning. I don't know where you live, but around here in SE PA if I were to go out and catch a mature pair of Bluegills today and bring them inside it would be almost a dead bang certainty that they would spawn within a month or so provided they're healthy, well fed, and don't kill each other. If you're in say FL, they've already started. As far as telling when they'll spawn it's easy. If you're used to the way the male looks normally, you'll have no problem telling when he's in the mood. His colors will really glow. Hope this helps.