Just because customers care about the problem, it doesn't mean they like your solution.
And as innovators you will be too biased in favour of your solution. Especially if you are not going to buy and use it yourself.
When you have a specific idea about your solution, how it will work and what it will look like, it is time to meet real people - potential customers - to get their response.
Right after step 4: Prototype, when you have some kind of visualization of your product.
To do a solution test you will have to show potential customers what your solution is. Not as a working and well designed product, but as something visual that will make them understand what it is in images. Not in words.
Preparation:
Presentation:
Steve Blank & Bob Dorf: The Startup Owners Manual. 2012. The method is well described in chapter 6, p. 227-256.