Mica caps are usually quite reliable, although I have had to replace a few in old TVs and boatanchor radios. I generally don't replace one unless I have first tested it, or have a strong indication that it's faulty.
Micamold caps are paper caps in flat, usually rectangular cases, and should be replaced. Some have colored dots, others have the values stamped in the case, something I don't remember ever seeing on a mica cap. I replace all of these without wasting time to test.
This article has a couple of photos and some guidance on how to tell one from the other.
http://www.antiqueradio.org/recap.htm
Note that the capacitor's value is a tipoff. Micas have very small values. Micamolds have larger values, much the same range as paper caps.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html