Next week I plan to attend an auction which has the estate sale of a collector of helmets. The collector and his spouse have both passed away. Consequently, the auctioneer knows absolutely nothing about the provenance of any of the helmets and is unable to authenticate any. I have attended other auctions with this auctioneer, and none of the participants has ever known anything about antique weapons as well (that is why I go). For all practical purposes, this auction has not been advertised to the public.
The helmets include morions, burgonets, and a couple that are of types that would go back to about 1200AD. Based on the pictures, they all look old. The auctioneer will let me inspect each item. Can anyone give me advice as to how to differentiate VIctorian replicas from authentic medieval and Renaissance helmets?

Realistically, you will not be able to learn enough between now and then to distinguish between Victorian copies and authentic medieval helmets. You could consider looking at photographs of known/extant medieval and renaissance helmets, but this will no enable to you discern with certainty. Unless you bring an expert with you, you should assume all the helmets are fake. Trust me, I've learned this the hard way with swords. ;)

Agree with Craig. Assume that they are all Victorian copies and don't pay any more than what these copies are worth. If the auctioneer thought that there was even a slim chance that one of the helmets was medieval then he would pay for a valuation out of his own pocket. A genuine 13th century helmet is worth many tens of thousands of dollars.

Thanks Craig and Dan for your good advice.
One antique dealer told me that there are so many good copies being peddled these days that unless the provenance of an item is known, it should be presumed to be a copy. I therefore keep a photo copy of the documents showing provenance taped to the back of the display case holding the one antique arm I own which has genuine historical rarity.
I went to the auction, but most of the prices were much higher than I wanted to pay. Most were higher than for similar items on eBay or Guntrader.com. English and Spanish Morion helmets went for about $1500 plus fees, and were believed to be genuine.
I wound up buying a spiked "Prince Albert" type helmet from East Lancashire regiment stationed in Egypt in very good condition for $180 plus about $50 in fees and taxes.

You should keep in mind that Ebay is notorious for selling fake artifacts. So those "cheaper" ones seen on Ebay are just cheaper fakes. But when you pay money for a fake thinking it's the real thing, even if the fake costs less, you still lose.

Even provenance of an item is not adequate to guarantee that you’ve got a genuine antique. The sword might have belonged to a famous collector, but how do you know the collector was not fooled by a fake antique? For that matter, someone can claim that the helmet was found buried in Bavaria, or dredged out of the Danube, but how do you know that’s true?

The best option is to try to get the opinion of several people who are known-experts to comment on whether they think the piece is authentic or not. But keep in mind that this is not a guarantee; all it can do is help you to have a better chance of weeding out fakes. To my knowledge, the only way to have a fairly reliable determination if the helmet is genuine is to do metallurgical analysis. However, these are costly and involve mailing the helmet to one of the few places in the world that can do the analysis. Certainly, this is not an option when looking at a piece in an auction.

So, be careful when you buy.

If you are talking about the estate sale in Anaheim on Monday, I saw some of the items online. It didn't look like you missed anything special. Similar pieces go up for sale a couple of time per year. Just go with a company that has a good reputation.

Yes, this was the Anaheim auction. A few years ago I bought some nice Luristan pieces at good prices there. That was before they had any links to online sites. Those make it difficult to get any kind of a good bargain.

FWIW "found in Luristan" is almost always shorthand for "illegally looted from somewhere in the Middle East but we aren't going to tell you where."