Depends on the auction, and even the better ones aren't totally safe. I've seen smaller auctions salt in a few bad knives to get rid of them. There was a famous mid sized one that was famous for passing bad Bowies, etc.... The truth is, most auctions either rely on the owner's description or a quick google search when it comes to knives. Only the super high end, famous collections get more than that.
If you're just starting out, buy through trusted dealers till you get your bearings or if you want to try auctions, try J Bruce Voyles. He runs antique knife auctions every few months, and I've never seen him be shady with anything.