I want to know if my AP is using 802.11ac or 802.11ac-draft. Can this be determined from information in a beacon?
No. Beacon frames are essential to the operation of 802.11 networks, but they are not very complex and they must continue to provide backwards compatibility with older devices.
As such, any changes to beacon frames are generally relatively minor and simple, making it a change that is both early in the process and agreed upon quickly. I am not aware of any 802.11ac product (draft or standard) that utilizes a beacon frame that can be differentiated.
Differences between drafts and final versions of the standards typically center around technical aspects (often of new features) and areas of contention. In many cases, the differences will be only in optionally supported features.