Ask about cribbing. Her neck appears to be a little built up, muscle-wise, underneath, and I see an electric wire on top of the fence in the background. Cribbing is a habit that is almost impossible to control. Take a look at her front teeth. A chronic cribber will likely have pretty short front teeth. Noticeably shorter than your average horse. Look for marks high on her neck and at the poll from a cribbing collar. If you ask the question directly, few owners will outright lie about it. Most will hum and haw and try to minimize it, by saying something like, "Well, she likes to chew sometimes" or something like that. If they give any indication that she may be a cribber, or if your inspection indicates she may be a cribber, pass on her.
In looking at Paso images on google, though, it appears that some Pasos just have that trait, so maybe not.
I always like to ask a simple question: "What bad habits does he/she have?" It may not get all the truth, but it normally gets enough hints that I can figure things to look for.