Electrolytic capacitors can degrade over the years. This problem cannot be detected with standard meters but a Equivalent Series Resistance Meter can detect this. See this wiki for the details.
Last week I have build this meter from parts that I already had, so it’s a little bit different then other ESR meters on the web. The design is based on Marvin Smith’s tester. Most changes were made because I didn’t have a TL082 but a LM324 as quad op-amp.
When it was finished, I tested my spare electrolytic caps (about 800). From these I found 15 faulty ones, mostly from the same brand.