How can I know whether a potato is too old?

It likely depends on what 'turning bad' means ...

If you have a couple in the bag starting to sprout, but the rest haven't, you can roast or bake the ones that haven't sprouted, let them cool, then store then in the fridge so you can pull them out to use them in something later in the week. (eg. home fries, patatas bravas or a hash).

For those that have started to sprout, but are still firm, you can cut away the sprouted bits (this time of year, you might even be able to plant them), peel them, and then boil them and turn 'em into mashed potatoes (which you can then vary for the next couple of days ... you can mash 'em with other stuff to make a sort of potato salad; you can add cooked greens to make colcannon or bubble and squeek; you can use as a topping for a cottage pie (the technically correct term for shepherds pie when you're not using mutton or lamb)

Some of these freeze well ... I've made up cottage pies and frozen 'em in oven-proof containers; you could likely do the same with just mashed potatoes -- I see 'em for sale in the grocery store all the time.

If you're looking for something to cook that just uses a lot of potatoes (in a non-whole state, in case you need to cut away parts) ... potato salad, potato bread, potato curry, latkes, potato soup, tortilla de patatas ... the list goes on.

... and if they're soft and squishy, or oozing liquid ... pitch them. They're rotting, and not worth getting sick over.

What type of potatoes are you using? Floury ones, like russets, will work best if you're frying them. For natural cut fries, julienne your washed potatoes and allow them to soak for at least 4 hours. Drain them, dry them, and blanch them off in peanut oil (assuming you have access to a deep fryer) for about 2-3 minutes. Drain, and refrigerate until cooled. You can use these blanched ones to fry off into nice crispy fries within 1-2 minutes. I've also blanched them in shortening, but the peanut oil lends a lighter, more appealing color and flavor.

For McDonald's style fries, a very effective recipe can be found on seriouseats.com, under the title "Perfect Thin and Crispy French Fries." I've used it a few times with amazing success;Say what you will about McDonald's, they have incredible fries.

Using the remaining good ones, pre-package anything you can think of in the freezer aisle ; hash-browns, home fries, etc. Then freeze them following storage guidelines.

The sprouts can be just rubbed off or cut out. Anything with black inside or outside, toss it. Potatoes should last several months, if stored right. Thus, in a dark and cool area, to prevent the green stuff. Google "how to store potatoes" for more information.

I have grown potatoes in my garden for years. In my experience, soft potatoes aren't necessarily bad, it merely means they will be bad soon. They should be used as quickly as possible.

I read in several posts that green potatoes have gone bad. I'm not certain if they were speaking about a green mold or some other green substance that has appeared on the potatoes since storing or if they were green to begin with. With that said, green potatoes that are green when you purchase them are not going bad (however, green skin is bad to eat, explanation later).

These green potatoes are not rotting or spoiled--it simply means that the potatoes were exposed to sunlight while they were growing. This causes the exposed skin to turn a greenish tint; otherwise the skin looks and feels normal. These potatoes have not "gone bad" but you do not want to eat green skin, as it can make you sick or even kill you.

Solanine, a natural glycoalkaloid, can occur when potatoes are exposed to too much light. The green color just under the skin strongly suggests that toxic build-up may have occurred. If you notice a slight green layer just under the potato skin, cut away the green portions of the potato skin before cooking and eating.

Again, these potatoes will be green when they are dug up or purchased. They don't normally "turn" this color after sitting in your pantry for too long.