IMO the bigger the better...meaning they're easier to sex when they're larger.
Male: Longer dorsal and front loaded look to them. Will have more bulk to its front end. The dorsal will be long enough to touch the top edge of the tail fin.
Female: more oval shaped body, and less colorful. Dorsal fin won't have the "whip" type length to it.
In a tank of GT's it'd be a good bet that the most colorful are males, but your best bet would be to look through the GT GoldSaum thread in the sticky section of this forum. There's a pic of a female I had in the first couple pages, and lots of males in there too. That should help a bunch