Yes, that's the rub for sure. The same question is also asked by non-technical people when trying to find a developer - how do you find a good one? So, I think you can follow some of the same principles: What projects have they done in the past? What results have they produced? Can you show some examples of your work? Can we speak to your references? etc.

Although it's not "turnkey" as the OP was asking about, you can do a lot of it on your own, or at least put together an overall plan, and then use agencies to fill the gaps. Just as with starting a start-up, there is a lot of great material and thought leadership out there on how to do it, along with a lot of practical advice.

Other than a few courses as part of my business degree, and working with marketing agencies in the past, I don't have a marketing background. However, I took a deep dive into marketing over the past 18 months, and we achieved some solid results. I spent a bit of time doing some research, came up with a strategy and overall plan, and then executed. Specifically, we put a content marketing program in place, along with some lead tracking tools, spoke at a few conferences, did some webinars, etc., all of which led to deals in the six figures. (In the B2B space.)

PM me if you'd like more detail re: resources.