Walk away from guarantees
Nobody controls Google rankings or advertising auctions. An agency guaranteeing a position or a specific number of leads is either inexperienced or hoping you will not check.
A credible agency guarantees the work and the reporting, not the algorithm.
Ask who will actually do the work
Pitches are often run by the founder and the work by someone you never meet. Ask to speak to the person who will manage your account day to day, before you sign.
Insist on owning everything
Ad accounts, analytics, domain, hosting and creative source files should all be in your name. If an agency resists this, that is the whole answer.
Look at reporting, not case studies
Case studies are marketing. Ask to see a real monthly report with the client details removed. If you cannot understand it in five minutes, you will never use it.
Start small and time-boxed
A three-month scope on one channel tells you more than any pitch. You will learn how they communicate, whether they hit deadlines, and whether they tell you bad news early.
The best signal of all
An agency that tells you not to spend money on something is showing you exactly how they will behave when it matters.