[exray]

The mica caps will generally appear as small plastic blocks with colored dots on them. Sometimes red, brown or black package. Sometimes called 'domino' capacitors because of their appearance.

As for replacement....touchy issue. They are generally more reliable than a paper capacitor but their failure rate is increasing as time goes on. They are most often found in the rf stages of the circuit and may have oddball values that are difficult to find and are critical to the circuit. I generally leave them alone unless I find a need to replace them. One of the main causes for failure these days is a ham-handed repairman twisting them over to see what is printed on the other side. That breaks the seal where the lead exits the device and often 'twists' the guts internally.

One exception when it comes to such caps. You'll see some larger values, say over 5000pf (.005) that have paper construction internally. They are as bad as any paper cap. A notorious use of them is as AC line bypass. Usually a large one, like 0.047 mf. Definitely replace that one straightaway!