Cwing For cantaloupe you sniff the stem end. If you get a strong cantaloupe smell and doesn't look rotten on the outside you should have a good one. If the smell is faint, it's not ripe.
For avacados, I just look for some that don't have dark spots. If you need them right away, you kind of need to squeeze them a little to see if they're getting soft. I just pick the best looking ones since I don't like squishing fruit at the store and if I find they are hard when I get home I just set them aside for a couple of days until they soften up.