I think some people are inherently better than others at sensing what is 'level' but I don't think those people are necessarily better shooters, but I've learned that I have my own internal bias (I usually cant clockwise a hair if I don't check) and I'm the sort of person who'll argue with the level then go home and check it against a weighted string and it's right, I'm wrong, but almost always in one direction by a consistent amount, and it's never bothered me because of that. Then I started trying to teach other people to shoot at distances (mainly my kids but there are others). I immediately noticed that people would fight me back and forth to cant their rifle an amount that I could eyeball as being grossly wrong while standing several feet away. I realized that in the future I needed to be more aware of it and once I realized what a problem it was when multiple people were shooting the same rifle, it really opened my mind a bit.
You can buy a cheap clamp-on level on Amazon that is light years better than nothing. I have one rifle with a level built into a ring and I have never been able to make it level, with the turret level and the reticle aligned with a level string (plumb bob) in the distance. I've learned to shoot that rifle a half-bubble to the right and never buy those rings again.