How can you tell if a female bearded dragon is going to lay eggs?

Well, assuming that you know it's a female, and that you know she's been fertilized, and her belly has assumed the much greater size than normal--she's laden with eggs--she will naturally start to dig underneath the substrate to bury the eggs. This can actually be a bit confusing, because Bearded Dragons tend to dig a lot, anyway. Best bet: keep your eye on her without disturbing her, and watch for unusual (for her) behaviour.