I echo Nick’s sentiment. Think of an agency as an employee that needs supervision. They might be experts at design or the technical aspects of how to run a facebook ad, but they won’t know your target audience as well or as deeply as you know your customers. You’ll have to manage them and really educate them on how you’re different than your competition. You should establish KPI’s (Key performance indicators) with the agency which will determine “success”. These should be as specific as possible. It will protect both you and the agency from disagreements in the future. You will have to determine what you want them to accomplish. Are they tasked with raising awareness of your business within a certain region? Are they tasked with driving sales? Each goal will have a different metric by which they can be measured. Some KPI’s to consider is ROAS (Return on Ad Spend), Customer Acquisition Cost, Cost per Click, Cost per impression. You’ll want to have 3-5 KPI’s to measure them by and give them goals to hit. Make sure these align with your broader business goals.