Basic checks to determine if used trans. works, before installation.
So far have done the following: no large particles at fluid pan, new front/rear seals, flushed trans., pan gasket.
1981 TH350auto. trans.
If you don't know the history of the transmission and you didn't overhaul the unit then it’s a gamble.
Clean red fluid and no metal in the pan are good signs of good transmission health.
You did all you can do for now, just install the unit and test drive it.
Make sure you have all three shifts, down shifts and manual shift the transmission to see if you have all the shift points D, 2, 1 and if the engine brakes (car slows down and engine races up) when manually down shifting the unit.
If your transmission has a lock up converter solenoid make sure the converter locks up at the required speed.