With action fully assembled, keybed clean, spring out, cheekblocks bedded, and glidebolts off keybed, reach through dampers, strings, and action parts with a long screwdriver (or other device), rest tip on backrail cloth, and tap. If there is a gap in the fit, a high-pitched tap should answer back. Try 2-3 places in each section.
Testing on backrail cloth will keep the screwdriver tip in place and reduce extra noise. (It's also where the keys land!) Tap over the backrail's underside lip, if you can. Going on the wrong side of the capo bar in the high treble can force tip away from lip on an angle that creates a false positive. Various obstructions make access awkward, but you can find a way through.
If the backrail is cantilevered, testing will be compromised. And tapping hard on a well-fit backrail may produce tap-like sounds not bedding-gap-related. Don't sand the gap that's too big to tap. And don't sand for a gap that isn't a gap!
Note in the photo below, the lip is at the proximal edge of the backrail (toward the player), instead of the more normal distal edge.
One difficulty in assessing backrail fit is psychological. We really don't want to face the bother and extra work of fitting the backrail, i.e., we really don't want to hear a telltale tap. We might also be insecure about our ability to recognize the problem and solve it. Therefore, we might tap a little too lightly. Or we might try our test in too few spots. Either way, we might miss the bad news we're there to acknowledge.
If there are places that don't fit, the topstack and keys must come off and we must begin a process that involves patience, time, and sandpaper. Well, maybe not sandpaper (more on process and alternatives later).
Some noise and instability from not collecting and acting on backrail data may not cause action failure, but they can dampen player inspiration and interfere with the music.
Next week: Bedding the Backrail