Your shop has such variety of fruits, for me water melon was always favourite, since I was little my parents taught me how to choose the matured water melon. You need to knock on watermelon and it resonates loud sound that means it is matured but if it is dull, that means it is still need time.

I do not know if it is scientifically proved but still using this method and in my opinion 99,9% I am right (from another side I sometimes think probably if you open up other ones probably they all matured :)