Dimorphism is difficult to notice unless the fish are at full breeding size. Males have a flatter, more levelled slope to their foreheads and are slightly wider about the head. Females have a more rounded forehead and are slightly plumper at the mid body when in spawning condition. There is no noticeable difference in odontodal growth in either gender.
As far as I know, golden Nugget plecos are where very hard to breed as they are quite territorial with each other, I have only heard 1 breeding success where they were bred in a 100+ gallon tank.
If your looking to bred them good luck and I hope this helps.