Roger
It'll take some very serious practice on an individual's part to play this game. Look VERY CLOSELY at the leading ray of the pectoral fins. What you will notice is that the front ray is particularly larger or thicker, as the case may be, than the rest of the fin. If you're still not sure, even the breeders will do this next step to be sure, feel the fin. You will feel the thickness if it's there.
To practice, look at some larger KNOWN females and males. Look for these indicators so you can train yourself better on what you're looking for in the smaller fish.
DISCLAIMER: This is not ALWAYS the case, but is about 90-95% accurate! Good luck. Let us know if this worked for you!
Mike