Watermelon: Many people swear by the "
thumping"
technique, where you thwack it to hear how it
sounds. A hollow
sound is good. But it'll sound
different depending on where you hit it, so it's better for
entertainment than as a practical
technique. It should be
heavy for its size (it'll be more
juicy). But best is to check the lighter-colored patch where it rested on the ground. You want it to be distinct and
yellowish in
color.
Cantaloupe: By far the best way is to smell the stem end. It should smell like... well, cantaloupe. Often, it won't smell like much of anything, which means it's pretty much tasteless. Or that your nose is clogged. Watch out, though, if it smells too strong -- it may be rotten.