Ask for a sample audit report first. A useful report ties each issue to a WCAG success criterion, names the template and component, rates severity and says how to fix it. Automated scanner output with a logo on the cover is not an audit.
Check they test by hand, with assistive technology. Automated tools catch only the machine-checkable share of issues, such as missing alt attributes and low contrast. Ask which screen readers and browsers they use, and whether they test keyboard-only. On a Shopify store, the variant picker, cart drawer and collection filters are where manual testing earns its fee.
Confirm they fix, not just report. Some auditors hand over findings and stop. That works if you already have a theme developer, but ask up front, because remediation is usually the bigger bill. For most stores one team doing both is simpler, and the person who found an issue can verify the fix.
Ask how they handle apps. Review widgets, popups, chat and loyalty apps inject their own markup, and a consultant can't edit an app's code. They should tell you which failures belong to which app, so you can raise them with the app developer or replace the app.
Scope by template, not page count. A Shopify store has a handful of templates (home, collection, product, cart, search, content pages, customer account) repeated across thousands of URLs. Testing each template, plus checkout and any custom sections, covers the catalogue without paying to test every product.
Red flags. A promise of "100% ADA compliance" or a certificate, because no consultant can certify legal compliance. An overlay widget sold as the complete fix. A quote with no defined list of templates. No retest after the fixes ship.