Bought a used car, how do I know what stall converter it has. If I put it in drive and drive it under normal conditions the RPM that it shifts itself at, is this the stall??

 

On the later models you can pull the gearshift selector into manual 2 and hold the footbrake with your left foot and step on the accelerator with you right foot and get the true stall of the converter. You may have to hold the brake firmly and be sure to go all the way to the floor with the accelerator. Just watch the tach and you'll have your true stall.
Kevin