I bought a used ceiling fan with a light and I want to test it for function before I install it. How do I do this?

Very carefully!
Mount it where you intend to install it with a proper box, properly installed per manufacturer's recommendations.
If it does not work, make sure the next one you buy can be mounted to that box.

If you bought it already just install it properly. It's not like it's that much work.
If it works, great. If not, take it down and start over.