New Prime or Ioniq 5

I suppose rust would still be visible if you checked without changing oil

 

Yep, but if I don't have to crawl under the car for some other reason, I will never do it.

 

I'm a big proponent of doing routine Caliper Slider Pin cleaning and lubing.
In fact Tesla has added that to their maintenance schedule.
Last few times I've dealt with a brake problem it was from a rusty slider pin. So I don't do it often enough....

I don't like doing it one bit, but jacking up the car and removing a wheel at a time will give you a good view of the bottom.
Creep on under there and have a look around!
I'm also a fan of spraying LPS3 or Liquid Film on any visible steel fasteners. And any holes or seams where the salty slush can get in.
I too live in salt country.