If your model was popular, there's likely a recap kit for its power supply. It usually makes senss to swap all the capacitors in the kit, unless the kit instructions say otherwise.

You can look for physical signs of degredation (bulgy, leaky, discolored), but to really test a capacitor for capacititance, you need to take it out of the circuit, at which point, you may as well put a new, high quality capacitor in.

The OEM capacitors may likely have a just right voltage rating, a new one with a higher voltage rating (and same capacitance, compatible type) may last longer in cirucit as well.

That would require some experience, yet the most common visual clue would be 'bulging'. There are some ways to measure ESR w/o desoldering but they won't be reliable at all times.

Measuring voltages, peak to peak, is a bit more work.