A ripe cantaloupe with be golden/orange in color underneath and within the outer rind. An unripe cantaloupe will be green underneath. Also make sure the cantaloupe is not too soft, a classic sign of being overripe.
The good thing about cantaloupe is that is will continue to ripen. So, if you pick an under-ripe cantaloupe at the market, let it sit on your counter for a few days and it should be ready to eat.