The concept of the Circle of Competence has been used over the years by Warren Buffett as a way to focus investors on only operating in areas they knew best. The bones of the concept appear in his 1996 Shareholder Letter:

What an investor needs is the ability to correctly evaluate selected businesses. Note that word “selected”: You don’t have to be

If you want to improve your odds of success in life and business, then define the perimeter of your circle of competence, and operate inside. Over time, work to expand that circle but never fool yourself about where it stands today, and never be afraid to say “I don’t know.”

That basic knowledge, along with some understanding of accounting and a little bit of study, would enable one to evaluate and invest in any number of restaurants and restaurant chains, public or private. It’s not all that complicated.