As said above. Also, if you press it gently, it should give just a LITTLE bit and spring back when you let go. BTW, you can pick it at any immature stage too (feels hard), but the tradeoff is smaller size.

Planter: Depending on the variety you are growing, there should be a color change in the skin as the melons ripen. But the best way to tell when a Honeydew melon is dead ripe is when the stem slips easily from the melon with a little pressure. Don't wait past that point. Don Yellman, Great Falls, VA

What variety are they Connie? Six inches might be too short for an Asian variety like Ping Tung or Ichiban, but it would be fine for the typical Black Beauty. Generally, the fruit should be glossy and firm to touch, if it feels mushy or has soft spots then likely you've waited to long. I'll paste in a site from University of Illinois Extension which has some good info on growing and harvesting eggplants. Here is a link that might be useful: U of IL at Urbana -- Eggplant ~ Watch Your Garden Grow

It depends on what you want to do with it, eat it soon or keep it for long term storage. Technically they can be picked and eaten at any time after they reach the size you want but they won't keep for long and the more you let them mature the better the flavor. But since most grow winter squash for storage purposes if that is the goal then you want to wait until the stem is brown and dry and the skin, the peel, of the squash is hard. That is usually in late August into Sept. depending on where you are located. Some use the thumbnail test - can't dent the skin with your thumbnail. Others use the pigtail test - the small curly "pigs tail" vine right above the stem is dry and brown. Dave