Cantaloupe: Smell it. It should smell like cantaloupe. It should also be slightly soft at the stem area. If it's green, you can leave it sit on the counter to ripen.
Watermelon: Knock on it. If it sounds hollow, then, it's a good one. The sweetest melons are cracked inside, and are ripe. When you are knocking, if it sounds solid, move on.
Green Grapes: Look for grapes that have a slightly yellow cast, particularly opposite the stem area. They are the sweetest. The really dark green ones are sour.
Pineapple: This fruit should be yellow near the bottom. I've also heard that you should be able to pull a leaf from a fruit that is ripe.
Now I'm sure there are many more tips that I don't know about!
What did your Mom tell you??