Two methods to self-assess the value of vintage beer advertisements are from print resources, such as Kovels' Antiques & Collectibles and Antique Trader Antiquities & Collectibles, or from online resources such as eBay. Both print pricing resources are available from their respective websites, larger bookstores, online book retailers and libraries.
Search for current and recent vintage beer advertisements on eBay to roughly determine their value. You can search eBay without an account, but you must have an account to buy or sell items. Search the brand name of the beer first, and then refine as needed to locate a comparable sign.
If you can't find a similar sign in print or online, consider a professional assessment from an antiques appraiser like Dr. Lori. Professional appraisers typically charge a fee per object being appraised.