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Really no way to test it. But you can look at the fluid. If it's nice and red that's a good sign. If it's brown and smells burnt, then it's got some miles on it and may need a rebuild soon. If it's watery, then time to rebuild.
Some transmission shops have a transmission dyno. I have never inquired about using one, but always wondered about it. Obviously testing them under load before installing them can save a lot of labor.