You'll know if they breed cause they'll become realy realy aggressive to the other fish. You say you have two of each species? Are they male and female or are you just assuming or hping they may be?
I'm hoping i don't know.. my dempsy stay togther like most of the time and my fire mouths are on opposite sides of the tank or ones at bottom and other at top.. i can't tell at all
Your dempseys will start diggin a large pit and they should dig to the bottom of the tank and they will change color and go darker. and like somone said become agressive. Make sure you have a cave for ur JD to spawn in. Mine spawned on the wall of a slate cave