This is the 64 dollar question for all prospective franchisees but you have a huge advantage since you're looking at the purchase of an existing franchise unit with an established track record.  Check the historical financial results carefully.  Make sure you look at trend data over the past few years and not just how it did most recently - find out if it is still growing and thriving.  The company you're looking at has a wonderful track record over a very long term of existence and that should help your confidence.  Find more detailed info about buying an existing franchise in the archived articles I've written at Entrepreneur.com.