old whats his name said:

I have heard that watermelons should have a yellow spot on the part
touching the ground. and they should have a certain thump when you tap
them with your finger.

Those are both signs, but here are two more.

Most varieties, take a look at the tendril nearest the watermelon. If it has
dried up naturally (not been bent or broken off) it's an indication of ripeness.

Feel the rind of the watermelon. When the melon is ripe, you will feel a
sort of ropiness when you grip the melon and draw your fingers over the rind.
On an underripe melon your fingers will slip across more easily.

Ripe watermelons will not slip the vine. They must be cut off.

And cantalpes must let go of the vine easily..

They will also change color under the netting and begin to smell like ripe
cantaloupes.

--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)