Google has lots of ranking factors - https://backlinko.com/google-ranking-factors
Google has many updates (about two a day!) - https://moz.com/google-algorithm-change
To say you are good at SEO is to say you can handle all of the updates for all industries for all ranking factors.
No business should promise that. No client should accept that guarantee.
If you wanted to measure 2 SEO indicators that would help you grow your business it would be
1) Domain Authority (Slow Moving indicator)
2) Link Velocity
If you understand how you rank compared to your competitors in these two factors you will easily move to the front of Google. For each industry/location/medium Google will change the rank of the ranking factors AND their Artificial intelligence tools will rank you differently based on what it learns over time.
If you are going to hire someone for SEO make sure the specialize in your industry and make sure they are aligned with your success.