"Reach" and "impressions" aren't the same as "customers"
Any agency can hand you a report full of impressive "reach" and "impression" numbers. Those numbers aren't wrong, but they aren't the goal. The only question that actually matters: how many real customers or inquiries came from this work?
Ask for a report from an average month, not their best one
Any agency can show you the best result they've ever achieved. Ask instead for a report from a regular month — consistent performance matters far more than a single exceptional result that happened once.
Ask: who is actually reviewing this work?
If you don't feel like there's a person who understands your business regularly reviewing the strategy, and the work gets handed off between a rotating team every month, that's a sign there's no real continuity in understanding your company.
Compare against the goal you agreed on from the start, not generic numbers
Before any partnership, there should be a clear agreed goal (number of customers, sales volume, brand awareness). A real review compares performance against that goal, not against a random "market average" that doesn't really exist.