There are many theories on using their coloration or body shape...unfortunately most are wrong. To vent the fish you would need to net it out and turn it over. Just google search it....
I have bred these (unintentionally) and the easiest way is to get a couple more and keep them together. Hopefully out of the group you would get atleast one male and one female. The downside to this is once the pair is formed they will try to kill (take that back not try...will) all the others in the tank. The male depending on the variety of jewel you have will turn a bright red when ready to spawn, whereas the female will usually only turn a slight pink with the deeper red just under the gills.
thanks i just got a JD last week and i put them together in my 45 and within seconds the battle started. luckily i hve another tank in my daughters room so i seperated them cause immediatly there was visual damage to the 2 of them. could there be a possibility that they are the samesex and they are fighting over territory or is it something else.