Read its Google reviews, ask for recent examples from businesses like yours with real enquiry numbers, check the agency’s published prices and contract terms, and search for the agency yourself to see how its own site and profile look. A trustworthy agency is open about price, process and ownership.
Ask to speak with the person who will do the work, not only the salesperson. Ask how many clients they look after and what a typical month of work looks like for a business your size.
Look at the contract before the pitch. Month-to-month terms with a short notice period mean the agency has to keep earning your business. Ownership of every account should sit with you in writing.
This is one part of a bigger picture. How to choose an SEO agency in Melbourne covers costs, contracts, what good SEO includes and how to spot a bad agency in one place.