A credible proposal should make the problem clearer before asking you to commit. Use the sales process to test how the agency thinks, communicates and handles uncertainty.
Ask what evidence shaped the proposal, which assumptions remain untested and why the recommended work comes first. A generic deck with your logo added is not a search strategy.
Meet the people responsible for strategy and delivery. Clarify which work is outsourced, how specialists collaborate, how much senior time is included and who reviews implementation before it is called complete.
Understand access, contract length, notice, deliverable ownership, data ownership, reporting, exclusions and the handover process. No responsible provider can guarantee a specific ranking controlled by a search engine.