I would like to general opinions as to the best way to test a transmission that is not installed in a car. I may decide to sell my spare transmission but want to know if it is good prior to advertising it.

Well you can put it into each gear manually and spin the input shaft, should be able to do that by hand, if there are any gears that free spin, or any crunching or it won't turn no matter what you do, then you have issues, other than that I don't know if any way to evaluate wear other than opening it up and looking at the parts.

You can drain some of its oil; if the oil is clean that's great!

If the oil has some fine metallic powder in it, that's not ideal but possibly ok.

If the oil has pieces of metal in it, that's bad.