1) Basically, with the lens set at infinity, look through the RF at something far away and see if it's "in focus". Doesn't tell you if the close-range adjustment (if the camera has it) is off, but it does put you in the ballpark and is a pretty good way of verifying accuracy.

2) Not that often: depends (hard knocks and airplane-type vibrations don't help), but some of my RFs are many decades old and spot on...