Usability Observation is a key aspect of Solution Validation. It can be used with any number of different prototype tools and throughout all phases of validation from early problem identification through to version testing while the product is in the market. To test usability you have to get users in room with prototype. Record the entire test as the user interacts with the software. Do not get involved as they use it. Encourage user to speak their thoughts and actions aloud and take note of screens and stages and the comments they have. If they ask questions be sure to note them and answer only what they ask so as not to influence the testing unduly. Follow up with a questionnaire about the experience and any thoughts they have on the problem, the solution or any other products or services they use in their typical problem solving process.
Over and over from beginning to end. Observing your users as they confront the problem you're trying to solve, whether within the context of your prototype or not, will always yield useful insights and help to validate or invalidate your assumptions.
The goal of both the Lean Startup and Design Thinking processes is to create ventures, products and services that keep the user at the centre of the design and the business. By testing products with users at every stage and without interference or interruption we get as honest an insight as possible in to the experience of the problem and the solution. This reveals the greatest opportunities for meaningful interaction and the areas that ventures can disrupt existing experiences and markets.
Your user, your prototype and your five senses - use as many as possible, quietly, to gain insights into your user and how they are experiencing your problem and solution.