Very hard to tell from those photos. If my life depended on getting a female out of that group, I'd go for the dark one in the first two photos - it looks fatter. Could of course be that it's just fatter anyways (males and females of P. maccus certainly grow quite fat given enough wood in the tank).
If the fish are settled in, well conditionend and "happy with life", then it should be fairly easy [I'm going by my experience with P. maccus - they have spawned a few times, but I've not had any fry (yet!)] - the males will be very hairy on the back half [or a bit more than half] of the body. If the fish just arrived in the shop, then they are most likely quite stressed (or have been recently), and they are most likely somewhat starved. Both starvation and stress will reduce the sexual differences in the fish.