Next see if there are any faeces in the aquarium. It should be either solid black or reddish in colour and always avoid fishes with white faeces.
As for body and head shape, colours etc please refer to "What is Flower Horn" section. If possible look for the features describe in that section.
Lastly people always ask us how to differentiate between male and female Flower Horn. If the fish is around 1 to 2 inches in size, then it is quite difficult to differentiate it. However bigger fishes are easier by looking at the fins and head. Bigger humps and longer tail-end fins are signs of a male Flower Horn.
Rounder bodied Flower Horns are also more sought after as compared to longer bodied ones. It all depends on individual preferences in the end.
Lastly we believe the most important thing to note is if you are happy with the appearance of the fish then go ahead and buy it. A lot of people always listen to other people's advices but you should know one thing, you are buying the fish, not them! We believe that if the fish is meant to be yours, you have a sort of bond with it and you know it deep inside you so trust your instincts.