For the most part, all the pedal problems that I have incurred have been:
(1) cable related. Cables shorting out, pulling out, bad input jacks
(2) Dirty switch. Not engaging or disengaging
(3) Power supply. Either dead batteries or faulty AC power. Some locations will experience voltage "brownouts" and "spikes" in the power grid and it makes pedals do strange things. Other things include: battery leads breaking, AC plug shorting out or lead broken, polarity reversal, loose/worn mini AC jacks
Knowing if a pot is bad is most easily detected on a unit that the pot is constantly in use such as a wah wah pedal. They will crackle or cut out. Simple solution is to use contact cleaner in the pot.
Other pot problem would be a broken lead or solder joint. If you engage the pedal while powered with an instrument signal and then shake it and you hear your problem then something has most likely come loose.
Sometimes a poorly responding switch problem can be power supply related. So before you replace the switch check out the AC connections and polarity, especially if you have other power supplies and pedals all hooked together. Again, try contact cleaner in the switch before major surgery. If the switch will not respond AT ALL then it might be bad. Do the other checks before replacing it.
Most mechanical/electrical related problems can be fixed with some good contact cleaner. Parts like switches and pots are somewhat durable and should last for many years. Usually it is the invasion of particulate matter or oxidization buildup or wear that causes these parts to under perform.
If you abuse your pedals, jump up and down hard on them, toss them into a milk crate along with all your power supplies and mike stand bases then your problems will require more in depth repair. Soldering then becomes a necessity skill. It's also going to end up costing you more if this is how you treat your pedals.
"No Tele For you." - The Tele Nazi
Ha! Tele-ish now inbound.