Watching them spawn is the best way. The one laying the eggs will be the female.
( Sorry
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As they get bigger, it might be easier to tell. Some males develop a hump shape on their heads but this is not true of all males. Females tend to be noticeably smaller than Males in a pair.
There is a way I use looking at the starting point and angle of the anal fin. This usually works but again, with all the new varieties and poor quality fish on the market, I would only use this on older strains or wild fish.
At this point, you would probably be best to just watch the fish. If they start to lip lock or chase any other fish away from them, they most likely have paired off and can become a handful to deal with in a community tank. Either separating them into another tank or separating them from each other will be your best solution at that point. For now, I'd just enjoy watching them as they grow