When do I know something is ready to harvest?

I live in Central Ontario, a cold continental temperate zone (Canadian gardening zone 5).
For the first time ever, I planted a muskmelon plant. It grew! We have 3 melons, but our temperatures are rapidly dropping and they aren't quite ripe yet.
When can I harvest them?

smell is important and also a slight give.
John S
PDX OR

Thanks, John. With frost approaching what I'm doing is an experiment.
Melon 1, the biggest, was picked last night and brought into the warmth of the house. It's pretty green...
Melons 2 & 3 had tomato cages put over them with clear plastic bags envelopping the cages to form mini-greenhouses. We'll see if protecting the fruit does help at all...

definitely smell, like John said.....some, and I don't remember which varieties, 'slip' from the vine when ripe with a gentle nudge.........I have read that when the first leaf at the stem wilts they are ready also....we are much farther south so have a longer season to ripen them. the melon we are growing this year, when ready to eat, gets yellowish and is fully netted and you can smell it when you walk nearby:) it can get overripe in a day in our heat.

Thanks Judith. I figured that it was the colour webbing becoming yellow and the fragrance, and I wondered if, like tomatoes, they pull off the vine more easily or not. So, number 1 was picked yesterday. I left 2 leaves attached and brought it indoors. The other two are now enveloped in their own personal little greenhouses as described above. We'll see what happens...

When the stem easily pops off of the muskmelon, its done.

Sort of off topic, but not really.. What about beets? How tall should the greens get before I start pulling them out? It's hard to tell when they are ready when you can't actually see them!

Hi Cassie, Beets will start to show above the level of the soil. Look for their "shoulders" showing. You'll learn by experience whether you want the little early harvest or a medium sized beet. Refrain from leaving them too long because they become tough. (Although I have heard that people leave them in the ground for winter harvest, but that must be a more southerly climate than here in central Ontario where I am!)
Debbie

Great tip! Thank you so much! I don't think the shoulders have started to show quite yet. I planted them in late July so they are just about ready though.

With beets, I tend to harvest the roots and greens together. You can take leaves here and there to eat without killing the plant/root, but understand that when you do this (or take onion tops, for another example), you are taking the part of the plant that collects the energy put into making the beetroot, so you will end up with a smaller root at the end than if you hadn't taken the leaves. You can harvest beets at any size - what I tend to do is seed pretty close together, do a first thinning like I would with carrots or parsnips to give the roots some room to grow, and then thin out the more crowded spots over the season, getting small beets at first and giving their brothers and sisters more room to get big. By the end of the season, I usually have some pretty decent beetroots, if the rodents don't get them first!

And up here in the north, I'm pretty sure you can't overwinter beets - they sit too far on top of the soil. Carrots and parsnips can survive in the ground (though I'm planning to mulch mine well before the ground freezes), and parsnips get sweeter when left in the ground overwinter.