How do you hire engineers whose job you know nothing about?

I'm the sole technical cofounder of a medical device company, and we're currently on the path to raise our Series A (have bootstrapped to human trials). Once those funds come in, we're going to make a few hires, mostly in engineering. My background is in electrical engineering and software engineering, and we'll be looking to hire an additional senior software engineer, an electrical engineer, and a mechanical engineer, whose main jobs will be to take ownership of their components and engage critically with our contract manufacturer, who will be taking on the majority of the actual engineering work.

My problem is that, in the case of the mechanical engineer, or really any technical discipline which I've not personally done before (for example, project management), I wouldn't know the first thing about how to evaluate their skills, and at our stage, the wrong hire can be a disaster.

Do most startups use talent agencies for this kind of work? I worry that our team's lack of technical prowess in certain aspects will lead to botched requirements and the wrong hires.