Buying a Used Fork--Request for Guidance

[lothian]

Turns out, the suspension fork on my mountain bike is entry-level, pre-loaded coil, pos. After a bit of research I'm persuaded a new fork is my Next-Big-Expense for this bike. To save money, I plan to shop used. I'll Google the various specs, features, tech of each make and model that piques my wallet. My present concern, though, is: How does one evaluate a used suspension fork, particularly in light of the fact that used forks are typically no longer installed on a bike.

Suggestions? Advice?