If you are fixing some used electronics chances are you will come across some bad caps that are causing some issues. Capacitors have a property known as ESR or equivalent series resistance. Caps will often test fine with a capacitor value check but are still bad. There are two types of capacitor ESR meters, in circuit and out of circuit. The in circuit meter will, as the name implies allow you to test the ESR of the capacitor without desoldering the device prior to testing. This will allow for fast testing and diagnosing of issue, many caps will fail with visible bulging but this is not always the case.
The picture above shows a nice and simple in circuit ESR capacitor meter, best of all there are full schematics and build details available for the design so you can build your own.