The ears usually tell how the horse is feeling
Ears Alert and Foward
Ears pricked alert and facing foward indicate the horse is happy and interested.
Ears Flat Back
Ears laid flat back against the neck show the horse is unhappy or annoyed.
Ears lowered slightly to the sides show the horse is relaxed, bored or could indicate that it feels unwell.
Flickering ears indicate the horse is listening and attentive.
Never approach the horse from behind, they can't see directly in front or behind, just from the side.
Let them know your there.
Watch the back end, because you never know if their capable of kicking.
And you also have to watch the front, they can strike out frontwords too.
Don't hand feed horses that are nippy (Usually they will tell you if the horse bites or not)
Really, you can search on google all this type of stuff
Type something like "How to tell how a horse is feeling" and some stuff should come up, or try "Horse bodylanguage"