How do you actually tell when a nonstick pan is finished, rather than just tired?

There is a test and it takes two minutes: the dry egg test.

Heat the pan on medium with nothing in it, no oil, no butter: until it is properly warm but not hot. Crack an egg in. Let it set for a minute, then push it gently with a silicone spatula.

• Slides freely as a whole: the coating is working. Keep the pan.
• Grips in places, releases with a push: tired, still usable for things that are not eggs. This is where your two in-between pans probably are.
• Welds itself to the surface: finished. No technique fixes this.

That gives you a category rather than a feeling, and it is repeatable, so you can retest in three months rather than wondering.

On scratches: a scratch is a place with no coating. A few fine marks in a pan that still passes the test are cosmetic and the pan works. What matters is when scratches become the surface rather than marks on it - food starts catching along the lines, and it spreads outward.

When it is genuinely done, and this is the one where I would not compromise: if the coating is flaking, blistering or lifting in sheets, or the metal underneath is showing over any real area, it is finished. Not because of a specific hazard claim, but because a coating coming off in pieces is going into the food and is not going to improve.

Worth knowing what actually kills them, because it changes how long the replacements last and it is almost always the same thing: heat.

Nonstick coatings degrade above a temperature that a dry pan on a strong hob reaches in a couple of minutes. Not searing temperature, well below it. So the fast route to a dead pan is preheating empty on high, which people do out of habit from cast iron.

The rules that actually extend their life:

Never preheat empty on high. Put the fat in first, and use medium.

Never sear in it. That is what a different pan is for. A steak crust needs a temperature the coating cannot survive.

Let it cool before washing. Thermal shock does real damage, and cold water into a hot pan is the classic.

Hand wash. Dishwasher detergent is abrasive and hard on the coating, whatever the label says.

Do not stack them bare. A pan stacked inside another scratches itself. A cloth or a paper towel between them costs nothing.

And on the replace-every-year advice: that is for someone using it hard and hot every day. A pan used gently and reserved for eggs and fish can go many years. The pan tells you when, and the egg test is how it tells you.

Replace everything or nothing is the usual advice and neither is a test.

The arrangement that made this a much smaller question for me: stop asking nonstick to do everything.

Mine now does exactly two jobs, eggs and delicate fish. Everything else goes in stainless or cast iron. The nonstick lasts years because it never gets hot, never meets a metal implement, and never does anything abrasive.

That also removes the anxiety about replacing them, because a pan doing two jobs is cheap to replace and you do not need an expensive one. The expensive nonstick pan is a slightly odd purchase - you are buying a consumable at a premium, and the coating on a mid-priced one is not meaningfully different in how long it survives being overheated.

Where the money is worth spending is the base: a heavy pan heats evenly and, crucially, is less likely to have a hot spot that cooks the coating off in one patch. Thin cheap pans usually fail in a ring where the burner is, and that is a base problem rather than a coating problem.

So my suggestion for your four: run the egg test, keep whichever passes for eggs and fish only, and let the others go. Then buy one decent mid-priced replacement rather than two, and use something else for the jobs that were killing them.