I know a few people who had some good years in the printing game (prontaprint and KallKwik were mostly franchised) and of course some, but by no means all, of the McDonuts are franchised.
The advice I would give is this.
Ask for a full list of existing franchisees, and you choose three at random to take as references. DO NOT accept the two or three pre-chosen existing franchisees which they will try to steer you towards. They may be shills, on a commission for any new franchisee that they persuade to sign up.
Ask for the churn statistics for the last 5 years - that the number of people surrendering their franchise, and then find those giving up and ask them why
Look online for a franchisee forum or user-group.
If you have any difficulty getting the franchise operator to co-operate in full with your researches, just pull out.