Similar to the User Feedback activity, the User testing is where you can start looking deeper into what the users do. While feedback is more about their general opinion, the testing stage is to observe how they interact with the product and what they are getting stuck on or what they are enjoying.
Be sure to watch closely and use the observation activity to help you. Look for small gestures and facial expressions. Ask them to perform certain tasks and record what they do. Was it easy for them to complete the task? Did the prototype break down when they expected a certain thing to happen?
You can do this with other members of the company that don’t have any contact with the project if you are strapped for time. If partners also want to organise this for themselves, make sure you are able to come along observe.