Some transmissions have a stand pipe.
You remove the drain plug and the stand pipe indicates if there’s enough transmission fluid. If a little dribbles out there’s enough fluid. If nothing comes out the fluid’s low. And as mentioned, you need a method to pump the transmission fluid up the stand pipe until fluid runs out.
But this is even better. Some manufacturers have a spec that indicates if the transmission fluid needs servicing. You hook a scanner up and compare the temperature of the fluid relative to the pressure. If it’s out of spec, then the fluid is replaced with tranny fluid exchange machine.
Tester