The Two-Finger Rule for Picking Ripe Watermelons?
The two-finger rule is a quick way to tell if your watermelon is really ripe.

You simply lay your two (adult) fingers over the dark area on a striped watermelon,

and if the stripe is at least big enough to contain your fingers, it's probably ripe!

Other Ways to Tell If a Watermelon Is Sweet or Ripe
I ended up finding four more easy-to-try methods of telling if a watermelon is ripe,

besides the two-finger method. Most of the authoritative sources out there

(including The National Watermelon Promotion Board

and the University of Georgia's extension service) include at least a couple of these.

.......... ...

They are: Round watermelons are sweeter and less watery.
Darker, yellower "field spots" indicate more flavor than lighter, whiter field spots.
A larger section of hatches (or webbing) indicates more sweetness.
Shinier watermelons are less ripe. Choose darker, duller ones instead.
Combined with the two-finger rule, this gave me a lot of ways to make a safer watermelon bet.

How to Pick a Watermelon at the Grocery Store

After looking over the selection, I decided on the watermelon pictured above. It fits the two-finger rule.

It was not as shiny as some of the others. And while its field spot wasn't the yellowest,

it had a lot of hatching on it.

WM's Additional Note: 
Check the Stripes: The stripes on the watermelon should be well-defined.

Dark green stripes should be deep and rich in color,

while the lighter stripes should be a creamy yellow.

Stem (꼭지) Color: If the stem is still attached, it should be brown and dry, not green. 

​A green stem indicates the watermelon was picked too early

                                                     
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