Item 19 (Financial Performance Representations) is usually where most potential owners will start, but I would emphasize Item 20 (Outlets and Franchisee Information) as an important indicator. A large number of franchise closures or even transfers from franchisee to corporate owned could indicate an issue that may not be obvious in the Item 19 (which isn't audited information and could be represented in ways that mask a problem). A rapid surge in owner growth could also be a potential red flag as the franchisor support systems may not be capable of scaling appropriately. At the very least, it can be something to ask the franchisor and other owners about in the discovery process to insure there isn't a lingering issue that could impact your success.
The most important item in a Franchise Disclosure Document is Item 19. This is where the franchisor provides financial informatio about the franchise and how it performs.