These are the primary way SEO agencies gauge results. Their mission is typically to improve the ranking for a set of chosen keywords. Being on the first page is the standard goal.

Unless an agency has developed their own rank checker, they will use a paid 3rd party tool.

The other form of measurement is simply looking at Google Analytics and seeing the improvement in organic visitors, site-wide or for specific pages.

Free Rank Checkers

SEOBase – from the people behind ZeroSSL. Free for one site and 3 keywords, as a taster. For $7/mo you can track 50 keywords across 5 sites, which is great value for newbies. No other tools, one-trick pony so far…

The HOTH – enter your website URL and see the highest ranking keywords it has, with the Google Rank. Powered by SEMRush. Free and easy, but presumably has limits

Paid Rank Checkers

SERP Watcher by Mangools. From $29/mo. Includes competitor and keyword research, and a backlink checker. Basically a budget Ahrefs, confusingly using a bunch of different websites for one service.