Ask the tough questions of management. One expert recommends that you ask the franchisor whether they will let you out of their franchise agreement if you are unhappy? If they respond "Yes", they are lying and will say anthing to get you to sign. (Franchise agreements are enforceable contracts and I have yet to deal with a franchisor that will let you out of an agreement voluntarily).
Know Your Market
Do the market research and understand how the franchise fits into the competitive picture. Also, think about how the market is going to change. Franchise profitability may vary from region to region.
Have an accountant review the franchisor's financials and have an attorney review the FDD and the franchise agreement.