American cantaloupes have a weblike skin, usually a light green color with cream-colored "netting" throughout. Look for thick netting, and feel the fruit for firmness.
Press down on where the stem used to be, and see if the cantaloupe feels a little soft. This is a good sign that it's ripe. If you can hardly press down at all, it needs more time.
Give the cantaloupe a light shake to see if you hear any seeds rumbling around. That's a sign the seeds have fallen, meaning the cantaloupe is no longer fresh.