Adapted from my answer on Quora.
Q: How do you find, hire, and work with a product designer?
As a hardware product designer myself, I can say that it's really important to be working with someone with whom you have a shared understanding of the product development process.
I've generally been lucky, but from time to time I have worked with clients who simply have a different idea about how hardware should/can/will be developed. While I tend to think that I'm right when it comes to these things, the bottom line is that it doesn't really matter: conflicts of this type are undesirable and difficult to resolve once you've entered into development.
Like with most things, it's up to the client to know what questions need to be asked. If it were me, I'd get smart in as many ways as you can. Start by going to hardware design/development/production meetups. Download a copy of Autodesk Fusion 360 and start modeling your ideas. Learn about as many manufacturing processes as you can, optimally visiting actual shops in person. The more you know, the better.
I would also recommend that you approach a designer with sketches and good specs for what you want to do and how you want it to look. In other words: if you have perspectives on these matters (some people don't, but more often they just don't communicate them effectively), you want to be clear on both design intent and aesthetics.
Something to keep in mind here: The more you bring to the table, the more clarity in your relationship with the rest of the development team, AND the less time & money you spend.
I would also, for what it's worth, take Tom Wolfe's "Man From Mars" stance, described here.