How to identify bad rocker/lifter?

[Bullochdawg]

What is the best method to identify a bad rocker/lifter and which one?

Saw a vid that showed disconnecting igniters one at a time to see if 'tapping' goes away to narrow down which cylinder is tapping.

Assuming this works, and once I have the valve cover off, what are the signs to look for in a bad rocker and/or lifter? Will it be loose? Discolored? Should I just change it - or all of them - while I'm in there? What about valve springs?

So many questions...

Thanks