Removing bias from IQ tests is one hell of a challenge, but if we put that aside and only analyse IQ results from people from similar backgrounds, it definitely measure something, and it usually gives accurate results. Meaning your score would not change much by taking the test again.
IQ score correlate with someone general ability in pattern recognition, languages, logic, bias check and etc. It also correlate with grades, salary, lifespan. So, is that intelligence? I don’t know, but it is something.
I also think so. You can usually tell who is close to your own level, but anything too far off andthe signals for what is considered smart or dumb tend to blend together.
The one trait which usually don’t lie is the person ability to learn. Fast learners and autodidacts are almost always smart, and people who are slow to learn are almost always left of the curve. Other signals you mentioned are deeply rooted in culture and can reqlly give the wrong impression.