The part where the vine was attached needs to be dark and puckered, not green. The skin needs to have some give. A little soft.
If you bring one home don't store it in the refrigerator until a couple of hours before you're ready to cut into it and serve it. It should sit at room temperature until then.
Slapping and thumping hasn't helped me in the past. I even saw the guy running a large roadside produce stand in Cherokee slap a watermelon and tell me, "oh here's a good one!" It sucked. It was dark all the way around so the chart is right about the orange field spot. Plus, and this is harder to detect, it was veiny, White striations throughout the fruit and it tasted bland. I'm still trying to figure out if there's a visual cue to the veiny fruit but slapping it and feeling the reverberation is not a ripe melon, let's just say that.
Feedback from the sound should reflect less reverb. The "feel" of the sound shouldn't reflect back so much. If it absorbs your input then it is a ripe and should be sweet fruit if it also matches the visual guidelines above.
Dark green vs light green melons? I think those are varieties. I see crates of both but never mixed together in the same crate.
I can't speak to the webbing on a melon. I should pay more attention to that and see if it factors in to my own observations.