Selecting the fish in the fish store might be easier since the fish may be seen next to each other for comparison. Checking a few times for "practice" observing them in the lfs
might help too. There are always some bigger or smaller ones and as they get older you might have an easier time to ID the difference.
As an alternative option you may consider selecting male and female of a possibly more sought after specimen (worth more $$ if selling juveniles) in case you have any interest in breeding them in the future. It can take a couple years for good maturity for breeding usually. Some of them are blue, green and other varieties with cool patterns.