[zrobotics]

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).

[TheAdamAndChe]

+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

[walrus01]

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