Memberships can be evidence that the business has some permanence in the community. Other memberships such as Chamber of Commerce are good too. Choose a service that promises to spend a set minimum amount of time with your pet. Always insist on a get-acquainted visit with the sitter prior to your trip. Learn up front what else is included in the fee.
Some services charge for the get-acquainted visit or charge an initial registration fee.
Some charge extra for each pet over a certain number. Pet sitting is a specialty service tailored to meet your pet's individual needs. If you have only a couple of pets, and are seeking to economize, a pet sitting service may not be your best option. Then, use the telephone. I would call the day before you are leaving to confirm the pet sitter's schedule and make sure the emergency contacts are handy, and call again a couple of days into the trip, just to make sure everything is all right.