As several posters say, "you need to educate yourself by reading books and magazine articles and going to see originals in museums and gun shows or hire an expert to advise you." The Colorado Gun Collectors Association show last May in Denver had three original Hawken rifles on display. Two were being exhibited by auction houses for auctions later that year.
As far as the second comment quoted above, Hawken rifles are part of our pop culture now and well known in collecting circles. If you "stumble across a treasure that's undervalued", it's probably too good to be true.
Sometime back, we had some fun discussing some suspicious Hawken rifles at some reputable auctions. You can read the old threads here,
https://www.muzzleloadingforum.com/threads/do-you-see-anything-suspicious-about-this-rifle.108733/
and here,
https://www.muzzleloadingforum.com/threads/suspicious-hawken-rifle.111878/
The first link above was about a Hawken rifle that sold at a James D. Julia auction in 2016. James D. Julia has since been bought by Morphy Auctions and the links to the original item and pictures no longer work. That may have happened when the James D. Julia website was merged with the existing Morphy website or they may had the rifle returned and sale nullified. The discussion in the thread is still pertinent, though.
Both of these rifles sold way below market prices for authentic Hawken rifles, which in itself is a big red flag. That means the serious and knowledgeable collectors didn't participate. In spite of this, both rifles sold way above the value of a really good contemporary rifle, so someone likely thought they "stumbled across a treasure that's undervalued".
You can often go to most of the auction house websites and search past auctions to get an idea of the current market for a type of gun. Here is a search I just did for "Hawken" on Morphy's website. It has a mix of antique and contemporary guns and some of the Maryland school rifles that Christian Hawken made. But it shows you what type of info is available through the internet.
https://auctions.morphyauctions.com/auctionresults.aspx?searchvalue=hawken&searchby=3