You will never need a knife to get a ripe cantaloupe off the vine. Take the fruit in one hand and the base of the stem in the other, and give a gentle twist. If the fruit parts easily from the vine, it's ripe. It should almost fall off the vine in your hand. It should also have a strong "melon" smell to it, and should look quite orange under the netting pattern on the rind, rather than green. I grow the "Ambrosia" variety, and with them the netting pattern is so thin and the change from green to orange so visible that I can recognize a ripe melon from my kitchen window. I'd never heard the term "full slip" before reading it here, but the Ambrosias definitely fit that description as well.