Once you've found the “best” concept, you have to verify your decision.
Once you have a winning design concept, that concept must be validated.
The whole process of concept design is geared to help the design team find the best possible concept. But the design team is not the final arbiter of what qualifies as the “best.”
Validation in design requires the approval of the deliverable (in this case, a concept) by all interested stakeholders. In the “real world” this will include your corporate leadership (i.e. The Boss), potential and actual clients, sales & marketing people, other engineers, potential users, etc.
In principle, all these stakeholders have had input into the design process so far, so there should not be too much trouble “selling” the concept. However, this does represent a project gate or major milestone of any product development project. At this point, those responsible for the product's continued development have a chance to stop the project if it is somehow bad. Alternatively, this is when the leadership would commit support for the next stage of the project. Because this commitment can imply rather large expenses in time and resources, it's important that everyone agree that the project is likely to be successful.
In the limited scope of an academic exercise, one validates a concept by presenting it to those who are running the course or project.
In either case, presenting the concept is key. So to do this convincingly, you will need to prepare a product concept specification, which is a formal record of the concept design phase.
In the case of this course, you will present your concept in one of the project milestones, and the feedback you receive will constitute the validation by your clients/users. However, this should not stop you from also seeking other input from people outside the formal setting of the course. Such work must be reported in the PCS too.
Also, when the concept design is reported, it must be verified by showing how it follows logically from the system design and must reconcile and differences between the concept design and the system design.
If the answers to all these questions are “yes,” then you have validated your winning concept1).
If the answer to any of these questions is “no,” then you need to fix something.
The most important indicator of a good design is consistency across all the stages of the design process. If you can't get a consistent result, then you're doing it wrong.
This means that there is no actual documentation needed for concept validation. If you have a valid concept, then it will flow “naturally” from all the other stages of your design project. Your instructor will know you have a valid concept if there are no inconsistencies across all the stages of your project.
A description of the refined final concept, per Step 5 of concept design.