Oh this is like how long is a piece of string. There is no sure way, but there are hints to make and educated guess.
Once picked they don't get sweeter like squash,what you pick is what you get.They can only get juicier. The flavour can change but I prefer them fresh.. shop ones are picked earlier and just don't have the taste so keeping them a bit gives a better flavour but nothing like a home grown picked when ripe one.
It needs to be neither hard..under ripe..or soft..over ripe.
As it gets older it can start to develop cracks.
Go to the blossom end.Under ripe, it will not yield to pressure. Ripe, and the skin is fairly easy to depress, if your finger goes through, or has no resistance, or the skin splits it is past it.
Some varieties of melons slip from the vine easily when ripe, others don't.
The crack around the stem end is a good indication again on some varieties. It starts as a crack, then gradually encircles the melon so the crack is a ring around the stem end. At full crack it will be right. at half crack starting to get there, with just a start of a crack it is still immature. Again on some types.
Bear in mind not all melons do this though.
Some can be judged by their smell,it starts sweet,and the perfume get stronger as the melon develops but you do have to learn with experience when the perfume suits you, if you can smell acetone it is getting past it.Many varieties have no smell though and some even smell like cukes when ready.
In a netted melon, the one with a sort of raised netted pattern on it, the more raised the netting the better the melon.
Watermelons are a whole different thing, so presume you are not meaning those,as this doesn't work with them.
As I said, like a piece of string, there is no "one way suits all"method,as their are too many types of melons, the pressure test come closest.
XX Jeannine
