A lot of the systems for sexing the young fish we see in stores break down because of colour being their basis - when fish are in bare store tanks with their companions constantly being hunted down by nets, they drop all colour and try to be invisible. Some of the most beautiful fish in the hobby don't sell well because they show badly in stores. Get them home, and wow.
Honey gouramis (the real T chuna, not the lumpy hybrid often sold under the same name) are a classic. An hour after they're in the tank at home, there is no mistaking their sexes. But in the store, it takes close observation sometimes, even if you've had the fish before. Stressed, they're colourless.
I like to breed my fish, so finding both sexes is kind of key. The breeder's trick when the fish are too young or when there is no "sexual dimorphism" (ie, they know but there's nothing the human eye can see as a difference) is to buy at least 6 of the species. Your odds on getting a pair are good that way. So are your odds of having 4 males trying to kill each other over the 2 females, so it does lead to multiple tanks, or membership in a fish club where you can share out the fish you bought but can't keep longterm.
The best way to proceed is to first, expect nothing from the fish sellers unless they radiate interest in fish. You can fake knowledge, but interest is another thing easier to spot. Curious people learn things. But be the curious person. A site like Seriously fish is great for learning. You are best to find out the Latin name of the fish, because English names are often used for 4 or 5 species, if they even look similar. Read up on the fish that interests you, then maybe image search so you can practice sexing them. Then go back to the store prepared.
A lot of your fish can live for many years, so it's good to take a little time and effort to get it right from the start.