From MyHorseDaily, some good things to look for and ask for when shopping for a riding partner.  Check out this article here.

Some of the highlights:

Ask to see her in a stall (to see if she has any stall vices)

Ask to see being caught in a paddock

Push her nose away (does she yield, or brace?)

Observe her being led (does she crowd? lead quietly?)

Ask to have her led away from the barn (is she barn sour?)

Ask to see her tied in a variety of places

Ask to see her loaded into a trailer

Ask to have her feet picked up