Without restating the obvious: it's something that I had to observe in person to be able do reliably. No profile description helped until I saw a few pairs in action. They are also much easier to sex when side by side. A single specimen often doesn't display the traits by which to sex visually until mature.
There's also a great article by loiselle about guapotes in which he has photos and descriptions of various parachromis, including jags. Google 'guapote loiselle', you should be able to pull it up.