How Do I Pick A Good Watermelon And Cantaloupe?

Watermelon and cantaloupe are my two very favorite fruits in the whole world! I can literally eat one of each in one sitting. lol

My only problem is, I do not know how to pick them. Unless the cantaloupe are 'Pecos cantaloupe' I take a chance every time I buy one that it will be juicy and sweet. Same thing with watermelon, the greener, the rounder, the heavier it is, it will be sweeter? I have no clue. Help! If you have any tips on how to pick the best one of each, please share so I can continue buying them throughout the summer and finishing them by myself. lol