I highly suspected my female was a female based on behavior well before I could see any blue on her gill plate. Then right before she spawned I started to notice the blue... When she had fry was the first time the blue gill plate was obvious (my fiance couldn't see it until then).
Now she has been mature for 3~4 months and has spawned a few times. Her gill plate is even more blue than it was 2... or 4... or 8... weeks ago...
So that doesn't help much
but verifies there is hope! Once you've raised a solid handful of Dempseys and have seen a couple pairs 'fall in love' their behavior starts to become a give a way before other signs prove themselves... it's extremely interesting to watch them... make an educated guess... then let them prove me right or wrong...
Such a dynamic and educational hobby... we deserves grants...