This question comes up periodically and my answer usually envokes either hysterical laughter or utter disdain but which served me well over the years: observe the fish when it's parallel to the background and check out the dorsal fin (PetSmart light-blue background really helps): even juvenile males have a transparent, filament-like trailer on the very last ray. It's pretty hard to notice the first time, but once you find it you will see it every time.
IME, Bolivians pair off at a relatively young age. At one time I had a single confirmed adult pair in a 75g, and even though they did spawn, they were constantly fighting.
The usual advise is to get 5-8 juveniles (5 seems to be my magic number for a 40 breeder) and let them pair off. Out of my last 5 (3M + 2F) from PetSmart, a single pair was apparent the next evening: the 2 claimed the right 1/3 of the tank, 1F the center 1/3 and the 2 males divided the left 1/3 front and back corner. The male in the back corner was chased and eventually starved to death by the rest.
A chancier way is to try to identify all males at PetSmart and watch the interaction and their territories: you might be able to identify a single female that does not get chased away by a specific male. For whatever reasons, if I am able to spot a pair at a store, they occupy the right front 1/4 of the tank.
2 days after the PetSmart escape, you can just see the pair in the front-right corner: