Well, I'd say it's more or less virtually impossible to explain all the things you'd need to know to determine without a doubt whether the rifle is an original. Especially given all the carefully done fakes that are being made today and for the last several years.
Probably the best thing to do is be sure it doesn't say Made in Japan, or Italy, or where ever someplace on the barrel. Also look on the barrel for a makers signature or stamp. Might say Hawken on the barrel, or Vincent, or something else. Take a look, check it out with a magnifying glass. Remember that stamped names are regularly faked.
Also, if it brass mounted, it's probably not an Early Hawken, but for $695, just buy it anyway! If it turns out to be any sort of Hawken, or Vincent, or just about any true antique, it's worth the price.
And since it's a antique store, offer them 10/15% less!
If it turns out to be a well made nicely aged and faked modern made rifle, either shoot it, or put it up for sale here!
Once you buy it, put the pictures up here. Unbelievable great deals Do pop up from time to time!
John