have bought a 1971 5-speed tranny that I will install instead of my 1971 4-speed tranny installed in the car today(1971 911T 2,2)

I am not sure about the condition of the tranny and I have no former experience opening a tranny and evaluate its condition.

Instead of installing it straight into the car, I want to have it checked out. Can I do a rough inspection my selves? any good ideas or tips what to do and what to look for?

The 4-speed is good, what should I sell it for?

Thanks
John