How can I check if a watermelon is sweet or not when buying?
From a farmer who was in love with his watermelons. 

There are 2 things you want to check for when eating a watermelon A)ripeness B)sugar concentration

To tell if a watermelon is ripe check the pale spot on it’s belly. It should be yellow/white not green at all. 

To test for sugar concentration tap on the watermelon, a dryer sweeter one will make a hollow sound while a wetter, watered down one will not make much of a sound. 

That being said you don’t want a bone dry one as that means it’s probably old.