Dont laugh, but my dad was an expert. He would always get good ones.  When I go to the market its a crap shoot. He did a lot of knocking and slapping then would settle on one.  Something about fluid waves or something.  Well he passed away and didnt reveal his secret.  Any ideas?

I'm no expert but here it goes...

First get a dog (your's or neighbors) and slap it's belly.  That hollow plunk sound is what you're listening for.  Now start slapping harder plunk plunk plunk, and harder PLUNK PLUNK PLUNK, dogs don't like that [:D]

Now find a mellon that has a yellow spot from sitting on the gound (this is required).  Hold mellon in one hand and slap, and you should hear that hollow plunk sound.

A light "thump" with a finger can tell you if the watermelon is ripe.  Listen to the tone difference between two or more melons.  The riper melon will have a lower pitched sound.

Cantalopes you have to smell to decide.

Just an old farm boy gone bad.

Arock out.

Damn 56K modem, got knocked off twice trying to answer your question!

The simplest answer is - Buy your watermelon from farmers on the side of the highway! or in the Farmer's Market in town.  Farmers can't afford to bring in melons that are less than correctly ripened.

Cut out the TomThumb/Kroger/Albertsons middleman
and help out the really needy guy!

If you can't buy direct then:

Chaingun is correct about the sound and color of a ripe watermelon, as is ARock on the sound of a ripe melon and smell of a ripe cantaloupe.

Most varieties of melon will get lighter shade of green as they ripen and they will invaribly have the yellow spot where they were on the ground.

But think about the farmer outstanding in his field!

Eric The([u]Former[/u]FutureFarmerOfAmerica]Hun[>]:)]

When I was little, I used to go to the market with my dad.  I would first slap his beer-gut and then slap the watermelons.  When the sounds matched, that would be the one!  It's never failed.

You guys can afford watermellon. Wow, someday....

These methods also work for selecting women, BTW..

Now, do you compare the watermelon to the sound of your hand hitting your forehead? [;D]

Shouldn't the topic be, "How to CHOOSE a good watermelon?"
If you're picking a watermelon, wait until the 2 tendrils closest to the melon turn brown.
And if you really want to cut out the middle-man, plant your own.  You probably won't be able to eat them all but won't have the guilt of throwing 10 cents a pound away when you take them to the range.

I like my melons in a C cup