What's the conventional wisdom on NOS electrolytics?

[ppppenguin]

Definitely test before using. Hook up to an HV PSU via a resistor and meter. If it reforms quickly it's probably OK. I'd then leave for at least 24 hours on soak to prove the point.

A friend once tested a batch of NOS 450V electrolytics. Fairly small ones. He said that 99% were fine.

While old small electrolytics are usually pretty horrible, whether NOS or used, the big cans seem to survive well and will usually reform OK.

As Notimetolooz said, if it's one where appearance is critical and it's faulty then re-stuff.