The disqualifiers, from hiring-guide consensus and our own side of the table: guaranteed rankings or lead counts (nobody controls Google), accounts created under the agency's ownership instead of yours, long lock-in contracts with vague deliverables, reports that count activity (posts made, hours spent) instead of outcomes, and reluctance to name who actually does your work.
The subtler tells: no questions about your margins or close rate in the sales conversation, a proposal identical for every business ('the package'), and pricing as a percentage of ad spend. Which rewards them for spending more, not better.
The single best filter question: 'Walk me through what happens to my accounts and data if we part ways in six months.' A good agency has a boring, immediate answer. A bad one improvises.