Ask to see every backlink they build
A real agency shares live URLs monthly. A reseller shares a count.
Ask who writes the strategy
If the person in the sales meeting will never touch your account again, expect template work.
Be suspicious of guarantees
Nobody can guarantee a #1 ranking. A credible agency guarantees deliverables and effort, not Google's algorithm.
Check their own rankings
An SEO agency that does not rank for its own city keywords is telling you something important.