the hard/soft bits you refer to are the armour plating that give these fish their name (armour-plated catfish). There is differences between the males and the females but again this develops with sexual maturity so if you check too young you will mistake a male for a female. you need to be very careful when doing this as the fish will be stressing out a lot and will be attacking with its spikes which can cause a nasty cut (speaking from experence).
Looking at the under side, down at the naughty bits, is a tried and true method with plecos and there are those who claim to be able to sex some plecos at 3cms. Bristlenose are a bit harder than say, the L333. but when they are a bit bigger its possible. Again, all these traits start to change at about the same time, guessing before then is only that, guessing.
Fin length can be misleading, and varys a lot with bristlnose species.
and like littlebristlenose said, peppermints are very hard.
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