How I pick pineapples is to try to gently pluck the leaves out of the top- if the leaves are easy to pluck its a good one, if they are hard to pluck or don't come out at all they are not ripe.
What I have problems with is picking a good watermelon. Its usually not ripe enough. Any tricks?
I think the biggest things with watermelons is the spot should be a yellowish color instead of white or green and the less shiny, the better. You're supposed to be able to tell by sound when thumping it, but that's always been a crapshoot in my experience.