Valves get tired at different rates, you can get valve testers, they generally reside with techs. Often, the bias drifts and you can hear the wrong, but a rebias sorts it. What valves depends on the amp, some have loads of options, others are more picky. My fancy Ashdown handwired job could take most standard power valves, but other amps will toast inferior valves or exploded if you put high draw vavles in. So depends on the amp.
Preamp valves are easier, there is less variation, and you can plug and play, bit of swapping shows the state of them.
Other components that have a chance of problems are: all of them, basically.
Electrolytic capacitors are the ones with a limited lifespan, even if the amp is unused (apparently more so if it is unused for a period), but resistors and capacitors can drift value, plus sockets and switches can corrode.
What amp is it, and how old, can give a bit more specifics that way.