how to tell if fish are ready to breed

[fishy_guy]

how can you tell when:
-black tetras are ready to lay eggs
-platys are ready to give birth
-swordtails are ready to give birth
-guppies are ready to give birth
-glow-light tetras are ready to lay eggs
-bloodfin tetras are ready to lay eggs

[feeder_guppy]

Guppies get a gravid spot on their rear. Sword tails, maybe the same. Definatly bigger. platies get fat I guess. All fish should be fat, i would think.