It is very hard to spot a fit healthy gourami from a maybe healthy gourami. All you can do is avoid the ones that look iffy.
Check the tank for dead fish. Although it is common for fish to die in a shop tank from stress, if there are a lot of dead fish in there it could be disease. The shop workers should remove dead fish as soon as possible, but if there are sick fish they may be dying too quickly. Also check the fish in the adjacent tanks as they run off a communal filter system.
Examine the individual fish for marks on the body - things like red patches, torn scales etc.
Look for thin emaciated fish.
Watch the fish in the tank for anything up to half an hour. You need to wait till the fish forget you are there. Males will have orange on the tail end of the body as well as the fins; females will have a dark stripe down the side, even just a trace of a stripe in the yellow form.
Any fish that has even a trace of black colouring on the throat is male.
Look for a male that is swimming round rather than hiding in a corner.
Buy a fish that does not show any bad signs; if there is a fish with black throat markings, get that one; if there is a healthy looking fish with black on it throat that is perky and possibly challenging other males, get that one.
The safest way to buy honey gouramis is to take someone with you. When you have selected your fish, send your friend to get an assistant and don't take your eyes off your chosen fish. When the net goes in the tank they'll panic and you need to concentrate on that fish so you get the right one.