I design a lot of consumer electronics. You obviously notice the way they strip obviously, but you can run them against a file to see how hard they are if you’re trying to choose parts/suppliers. There are other things to look for, like how precise the slot is or how smooth the outer edges are, but that’s not as important. IMO stress (shear, stretchy) tests are almost totally irrelevant for consumer products that have plastic or soft metal enclosures which will deform far before a fastener will break.

Let's say you are given a product, which you can disassemble as you like. can you give a reasonable estimate of lifetime this way ? or does it require prolonged stress tests ?

And if estimation is possible, how does one learn how to do it on their own ?

This is more of an experience thing, but it is usually very obvious if something is poor quality. A good thing to look at is typically the plastic parts. Look for signs the mold was in poor condition or cheap such as excess flashing or surface roughness. Another thing to examine is how thin the parts are, and presence or lack of reinforcing ribs. For comparison, take apart something like a Milwaukee cordless tool vs. a cheap no-name equivalent. AvE on YouTube does these kinds of teardowns, and does a decent job of explaining design decisions, but is definitely NSFW.

Stress testing screws would be pointless for what you want to achieve, but like the other commenter said you can look at them too find signs they are poor quality (irregular heads, bad plating, etc).

+1 for AvE[1]. He's NSFW compared to bible school due to cussing and occasionally dirty jokes, but he uses language that pretty much any manual laborer in the country encounters daily. His analysis of how things are designed have taught me all sorts of things about how tools are made.

[1]https://www.youtube.com/user/arduinoversusevil

I would also recommend this guy's youtube channel, he disassembles and examines both normally priced and suspiciously-cheap-from-china consumer electronics:

https://www.youtube.com/user/bigclivedotcom/videos

cheap consumer electronics can be assembled in a method with screws intended to be used once into a plastic cylindrical receiving part (injection molded usually). The screws have a certain sort of spiral to them that you don't see with screws intended to be repeatedly installed and removed. On the cheapest stuff, removing the screw once and then reinstalling it can result in the clamshell assembly of the upper and lower injection molded parts becoming permanently loose... One example would be a "modern" humidifier such as you can buy at Target that is never intended to be repaired or disassembled.

Those are self-tapping screws.

In the old days, items of wood would suffer from the same issue when disassembled, although it was then much easier to fill the hole with putty and redrill.

With plastic, I suppose you could use epoxy to fill the hole.