A lot of auctions allow you to walk around the horses before the bidding begins. This is a good time to check out potential prospects. Go up to them, pet them, rub their ears (if they let you), pick up their feet (if they let you). The horses responses to these actions are huge signs as to what their personality will be like. If they're mellow about it, easily pick up their feet for you, don't mind being rubbed anywhere, then chances are that's what their personality is. However, if they're extremely head shy, wont pick up their feet, and have generally bad manners, then that too could be what their personality is really like. Also, horses being drugged at auctions is not an uncommon thing. I've gotton a few horses from auctions, some of which were the best horses I've ever met, and some which I wish we had passed on. My friend bought a horse she thought was nice and calm and had decent bloodlines, turns out when she's not drugged up, she rears up and flips over on people. Oh yea, and she was pregnant.
I've spent most of my life riding horses. The rest I've just wasted.