The best way to test a product is with a high-quality prototype that allows the users and project team to really understand what is being made.
Now that the product is getting close to its final stages, it is time to start doing some high-quality testing. Unlike the rapid prototypes, these should be high quality and well thought out. Focus again on a single idea and design for it, sometimes these will make up a large part of the product that needs to be worked on. These prototypes can come in the shape of invision prototypes with a click through of screens. They are good to show the team exactly how a product looks when it’s done and how a user navigates through them. They also allow the target users to really give their feedback.
These prototypes can also include code prototypes. These are usefully for testing functionality and concepts before really fleshing them out into the full product. Use sites like Codepen or JSfiddle to test out ideas or even build them on Codebots and see how they work.