It varies GREATLY from auction house to auction house.
Most auction houses do not allow cameras, so any pictures you may find are usually taken against the rules.
I went to my first auction (RSD in Thurmont, MD) this past Friday. After hearing horror stories about auction houses like New Holland, Enumclaw, and Camelot, I was very pleasantly surprised. These horses all had their own stalls (four walls, not standing stalls) where they couldn't be bothered by other horses. They all had fresh water (even though it is the seller's responsibility to provide water for their horses, most people will refill a horse's water bucket if it's been dumped or soiled). They all had good-quality hay, and their stalls were very clean and well-bedded. I don't know how far ahead of the auction someone could bring in a horse, but it didn't look like any of them were at the facility for longer than 12 hours before the auction.
At RSD, there are very few kill buyers who ever frequent it, and there were none this past Friday, so all horses either went to private homes, a rescue, or horse broker. Most of the horses were of good quality, so even if they were bought by a broker, they're not going to be sent to slaughter, especially as prices were well above meat price. There were two Haflinger colts who went for $75 apiece, but a rescue won the bid. They were so small and underweight that a kill buyer would have lost money on them, even if he would have spent only $5 apiece. There was a big stink over someone getting kicked out of the auction this past Friday (they were there to purchase horses for a rescue, but had violated posted rules on numerous occasions, so the business owners were fully within their rights to ask them to leave), but most people don't realize that a horse going to this auction does not automatically equal the horse going to slaughter.
The horses are "run through" very quickly, and if there are a lot of horses that night, the horse being shown in the ring might not even be the horse that's being bid upon, so you really have to pay attention to what's going on. If a horse is broke and came with tack, it is ridden through (when I saw it) by riders provided by the auction house. It is walked, trotted, cantered, and galloped back and forth, and turns are usually sharp and designed to show off the horse's responsiveness.
Other auctions are not so nice. It's common for there to be a dozen or more horses crammed in a small pen for days with filthy or non-existent water, with little and/or crappy hay, on concrete with no bedding except for manure. I haven't been to New Holland yet, but depending on the time of year, a lot of horses are either bought by kill buyers (in the Fall) or by the Amish (in the Spring).