A legitimate monthly retainer produces artifacts you can point at: pages written or improved, technical fixes shipped, links or citations earned, and a report tying it to movement. Ask for the list every month. 'optimization' with no nouns attached is the classic empty retainer.
How to know it's working, in order of truth: leads and calls from organic (the point), qualified organic traffic trending up in Search Console over quarters, then rankings for terms real buyers search. Rankings for invented long-tail phrases are how bad agencies simulate progress.
The cadence of honest judgment: 90 days to see the machine running (artifacts shipping), 6 months to see the trend, 12 to judge ROI. If month three has no artifacts, don't wait for month six.