As part of my PhD, I designed a set of user interface design guidelines for an in-vehicle device. This device does not exist at the moment. What would be the best methods to test/validate these guidelines taking into consideration that we don't have a product to test them?
Do you have the opportunity to create a low-fidelity prototype of the product?
The Wizard of Oz experiment comes to mind as it works very well on unfinished products (specifically low-fidelity prototypes): it mimics a finished product by involving a human actor (usually out of sight) to perform actions on behalf of the system. However, this method requires you to have a clear idea of what the finished product is going to do.
By using this usability testing method, you can set up scenarios between your user and the system actor that will put your guidelines to the test (of course, this approach may not work with all of your guidelines). It does, however, work well in real-life situations: for instance in a car; where the system actor could be sat in the back seat, out of sight of the user undergoing the testing.
Usability testing paper prototypes works really well when you don't have a working system to test. I've used this technique to test small pieces of a larger system. ("Do people understand how the timeline on this scheduling app works?")
Its greatest advantages are that it's so cheap, and it's so easy to make changes and re-test.