You can use the BuiltWith Lead Prioritization feature to determine which sites have not been enabled for mobile browsers. One of the key elements that determine a site is mobile optimized is the use of the viewport meta tag, which is used by many mobile browsers such as Safari Mobile, Opera Mobile and Firefox for Mobile.
You can build a prioritized report that says “Show me all sites in my list that aren’t using Viewport Meta” as shown in the screencast. You could do the reverse (show websites that are using Viewport Meta) to determine which sites in your list were mobile compatible. You may also want to consider filtering by CSS Media Queries such as Max Width to determine which site are using and not using special CSS instructions that resize the window based on the size of the device accessing it.