How do you know when your fighting fish is pregnant?

That depends on the fish (i.e. whether it's an egg layer or a live-bearer). Guppies and sWordfish (for instance) develop a dark spot on their lower abdomen which swells as the babies develop. These fish are live-bearers. I breed fresh water angelfish. They lay their eggs in long swaths along plants or a favorite rock. I've placed several large pieces of slate in the aquarium that they seem to like to lay their eggs on. The female will swim up the slate and deposit her eggs and the male swims up behind her and fertilizes them. In a few days the babies hatch but stay "stuck" to the slate for about 72 hours until they're big enough to start swimming on their own. The adults will watch over them and protect them. It's kinda cool to watch. The babies surround the adults like a cloud. You know the female's pregnant because she'll start cleaning the slate in preparation to laying the eggs. It's difficult to actually determine that she's got eggs because she's so big to start with.