You can't tell by looking at them typically.
My mule Red is real friendly looking and calm in the pasture. Until yours 10ft away and he blows up and bolts as far from you as he can get. This bolting doesnt apply at feeding time haha

"Blessed are the flexible for they shall not get bent out of shape"
"You can discipline and Discourage or you can Discipline and Encourage"
"Gentle in what you do , But firm in how you do it"