I'm not sure, I just go on ebay and search the latest "sold items" of the item I have if I want a ballpark figure of value.
You probably could find a "proper" website to do a "proper" appraisal, but I don't think it'd be free...although I know Crocker Farms will give people a free appraisal on stoneware, so there might be something like that for bottles if you look hard enough.
Also you could always post a pic of the item in question on this site and we can try to give you some idea of what it might be worth....
I wasn't sure appraisals were allowed on this site. That happened to me on a Facebook group. When I asked for an appraisal (not bottle related) on an antique group, I was kicked off and blocked. What the big deal????