Yearlings have to be nominated to and inspected by the sales company prior to most sales. Especially at Keeneland September, the yearlings are already broadly "ranked" both by their inclusion in the sale to begin with and the "book" they are assigned to (i.e., the early books have the higher ranked prospects). So there is at least some assurance of potential quality there.
In addition, a lot of veterinary information is collected and made available on each yearling. Some of this is provided by the sales company and the consignors already, but most serious buyers tend to hire their own team of consultants to evaluate potential purchases.
http://www.salesintegrity.org/pdf/cba_sales_booklet.pdf
