I want to see the mule stand still to get on, waiting to see what the rider is going to ask of her. That could be step to the right two steps, or to the left five steps, or a quiet back up or back five steps, then walk off quietly. If the mule is saddled ask the owner to remove the saddle. Look at the mule's back. If you see white spots, she has had an improper saddle on.
Folks try and fit a horse saddle on a mule and they hurt the mule's back.
The bigger the white spots, the more the problems. Look for any old scars on the mule and ask questions about the scars. Other saddle fit problems are evident when the mule is walking short and choppy, jumping around, or even kicking the saddle out of your hands. Have the owner lead the mule back to her corral and turn her loose. Visit with the owner and listen to the stories about the mule.