I check to make sure the front of the saddle tree bars are behind and off of the scapula. I run my hand under the saddle and pad, find the shoulder and just eyeball the tree. I don't know if its all saddles but if mine is sitting too close to the shoulder I think she has a harder time lifting her front feet.
Of course all horses trip from time to time.
Here is a really good illustration http://www.horsesaddleshop.com/saddl...ing-guide.html