Be sceptical of guarantees
Nobody can guarantee first-page rankings or a fixed number of leads. Agencies that promise them are either inexperienced or relying on you not checking.
Insist on account ownership
Advertising and analytics accounts should always be owned by your business. If an agency refuses, that alone is grounds to walk away.
Check how they report
Ask to see a sample report. Good reporting explains what happened, what it means and what happens next. Screenshots of platform dashboards are not reporting.