Stand (or sit) motionless in front of the tank for several minutes, at least 15 or more. Observe how the fish interact. Remain as motionless as you can, because when fish see movement outside the tank they usually get sidetracked, so to speak, thinking of possible food, or danger. As you observe the fish interacting, the males should be pretty obvious, they will be continually "pushing" at other fish of their species. Once you see this, look for a male that tolerates another less-brightly coloured fish close to him. This is the easy part. Once the store person has the tank open and the net goes in, keeping track of the fish you picked out can be tricky.
What some aquartists do is to buy several fish and place them in a suitable tank and let a pair form. Problem with this is that you then have extraneous fish to get rid of, unless one has another largish tank for them.
You might have luck returning just the stressed fish, if it happens to be a male, and selecting an obvious female using the same method as above. The male you have might accept her. I did this once with the closely-related Bolivian Ram. The male I had accepted the new female, and they spawned four times. But I could tell right from the start that their relationship was rather tenuous. They tolerated each other most of the time, but not all of the time, and after the fourth spawning the male killed her. And they were in a 5-foot heavily-planted tank, providing plenty of cover for the female. But obviously not enough to suite the male who just wan't prepared to have her in his space...and this male owns this 5-foot tank and makes that very clear to all the other fish which are corys and tetras.
Byron.
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