With bolivian females it's a nightmare. When I went to get my four I have ended up with what I 'think' is 2 males and 2 females. I'm about 90% sure about one female and 80% about the other. I had picked them as 1 male and hopefully 3 females (based on physical characteristics at the store) and from behaviour + physical changes I've come to the conclusion of 2male and 2females. But I can only really be sure of the females being females if/when they spawn. Although my big male spent alot of yesterday being a fishy plow (so funny to watch, mouth open pushing gravel) and making himself a nice big 'pit' next to his rock. So hopefully one of the females will give in.
Only tips I can give for when you're looking is to look for a round stomach. And not just at the front (that's the actual stomach), but sorta broad at the front but still chunky towards the middle. Side on my females are a bit rounder rather than long in comparison to my males, not much of a nuchal lump (google it lol), and if you can ask for them to be fed that could help. Cause with full stomachs they tend to put their breeding tubes down. With males it's a little wedge on their underside. With females it's a thick tube. It wont neccessarily point straight down (although it is more down than backwards), but the point is it's wide and blunt. As upposed to small, pointy and wedge shaped in the males.