Stuff off the top of my head / things I would check before using a boarding service....
What experience/qualifications do you have? Are you a member of any professional bodies or associations (or do you plan to be)?
What insurance do/will you have? What does it cover? For example, does it cover one dog injuring another, accidents that result in harm to the dogs, a dog damaging your property?
What facilities do you have? e.g. outdoor or indoor kennels, secure exercise field? Are you going to do pickups/drops offs? If so, what transport vehicle are you using?
Is it just you are will you be working with anyone else?
What's the maximum number of boarders you would accept at any one time?
What would a typical day with you be like for the dogs? i.e. what activities etc.
What do you plan to do about time/off or holidays (e.g. any back up coverage or should owners make other plans)?
What if they are not gorgeous pooches but dogs that are trickier to take care of? How would you handle that?