The appearance of an old capacitor tells you basically nothing unless it has catastrophically exploded.
Caps may look perfect on the outside and be utterly worthless on the inside. Conversey, as ggregg noted, an old cap can also look awful on the outside -- all grungy, dripping, etc. -- and still be operational.
It's like finding a 60-year old car in the barn and trying to guess the condition of its engine by inspecting its paint job. Not a reliable guide
The failure rate for 60-year old electrolytic and paper capacitors is so high that you will save a lot of time and grief if you simply replace them all.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html