I haven't had much luck yet this year picking watermelons. My dad never picks a bad one, and he is contanstly trying to give me lessons on how to do it. So far, it has not sunk in. 
But a few days ago I was on the Food $ense Facebook Page and they had some tips listed on how to pick a good one. I am going to copy them here. But You should go check out their FB page!
1. It should feel heavy for its size.
2. Have a relatively smooth rind that is slightly dull on top.3. If the bottom of the watermelon (where it was resting on the ground) is cream yellow, not white or green.4. The top of the watermelon should be dull and not shiny.5. A deep hollow sound when you thump on the outside.
