Brake caliper question

I'd suggest if there is no evidence of the pad dragging nor fluid leak to leave them along!

With that said, Nissan in the 80's recommended a fluid change for ALL the fluids (not a flush) at 60k or 5 years.

BTW, when you replace the pads the service tech should let you know about caliper condition

Hachiroku's explanation is pretty right on but to answer the questions:

Brake calipers are checked visually and functionally to see if the piston(s) move freely and if the caliper has slides, that the slides and sleeves move freely and that there are no fluid leaks.

Depending on caliper design, some calipers have one piston, while some have more. Our Sequoia has 4 piston calipers and no slides.

Calipers should be replaced or rebuilt if the piston doesn't move freely, if the slide or sleeve can't be freed up, if there are fluid leaks, or if the caliper has been physically damaged.