This might sound harsch to some people over protective of their little C64, but best practice is just doing - it with apropriate caution ofc.
Always check the inside for obvious dirt, dust, rust and damage. Remove if any. Next plug it in and power it up (keep the lid/cover closed). If the machine has been stored in dry, clean condition, it should be fine anyway.
There isn't anything you can't prevent without spending way too much time. If a cap is dead or some short happened, they will blow anyway. I did reactivate hundrest of machines over the years and basicly all just worked. The faulty ones usually had issues with dirt and rust, easy to detect before. Noe ever 'blow up' or caught fire. PS faults have been rather rare - and easy to detect when the breaker flips :)) Still, watch out for smell and smoke.
I just got two Sourcerers after 30+ years of rather sub standard storage. Quite some rust, but no obvious problems when checking the boards. Both worked right away. The biggest issue was to find a plug converter, as these units had UK plugs.
After almost 20 years, you might want to change the battery. At least if you intend to use the machine more often. It works fine with an empty one, but all setup (time, date, slot assignment) needs to be entered after each power up. The Battery is soldered in, so it needs some (easy) disassembly to change. AFAIR Digikey still got them listed. Use only (if possible) the original types (maybe of larger capacity), as look alikes may show different behaviour and reliability.