Our secret tip: to evaluate a potential service provider is this: Look for the company name or the names of the listed team members on the respective $cms platforms.
People and/or companies with a history of contributions usually know their software quite well. Otherwise they might have tried to commit a patch or two but give up when their work does not stand the coding standards.
You can find assistance on a general level of evalutions during our consultation hour. Our ambassadors are happy to help finding the suitable web pages, repositories etc.
As an alternative, ask your tenders to list and prove their software contributions in the bidding documents as a proof of professional capability.
Tenders without visibility in the community
This is not to say that service providers without community reputation do a bad job, not at all. It is just more difficult to judge.
Of course, you can ask for a handful of showcase projects. But you can still not be sure how much the modern design has to do with robust technology handling or whether the company always hires expert freelancers that are gone after the project was launched.
Why not just ask Google?
Throw the name of the CMS and maybe "agency" and the city name into the search engine and just invite the first three to a pitch? You can do that. If expertise in search engine optimization is the most important qualification for you.
However, in our observation, the ability to rank a $cms web page high does not mean expertise with the CMS's implementation.