Time is often in short supply at the end of a development project and it will not possible to do a major redesign if a fundamental fault is discovered then. The evaluation should be conducted against the previously formulated requirements. The performance measures specified will be of special importance. Preferably, the prototype should encompass the whole interface of the application, though this can be hard to achieve. In these, one can relatively quickly draw the interface and attach some functionality to it.
Testing can be formal or informal.
Informal testing is done as a sort of quick check of whether the design is valid, but reliable results can only be gained by formal testing which should preferably be conducted under controlled conditions and with the intended users. Read what others have written.