What is the best way to test a game prototype?

Being really clear on your goals is essential. What is the fundamental issue you wish to investigate? Are you proving a hypothesis around game play? Examining an area of technical risk? Testing a workflow? Focus on that. Do that. Especially in games, it's easy to draw in far more considerations than necessary and let scope creep distract you from the point. Challenge yourself if you think you are playing into your comfort zone: if you already know it can be done well, then skip it.

Your testers should be a good blend between people you know, people who knows the game as well as those who don't know your game. This will allow you to get a diverse and un- bias POV on how your game is performing. It's also important to choose people that are within your target audience, and if possible, even those who are slightly outside of it

There will be 2 types of testers, simply said, first, the people on the development team and second, the people to whom the game will be delivered i.e. the audience. The first one, known as alpha testing, will help us understand what new things can be added and where the bugs can pop up since each person on development team will have a different way to look at a problem. Second, beta testing, where the players will test the prototypes of the game and help us understand what more can be added, through the players' feedback, to enhance the overall gaming experience. Hence based on these 2 types of testers, we can improve our projects.

Testing my prototypes through video calls have been the best experience for me. It's easier for a tester to help you, if it's just a simple call where they share their screen with you. Recording the session is beneficial so that you can return to it later and recollect what happened. Finally, I would encourage the tester to speak their mind as much as possible while playing, while muting myself to avoid any interference. This allows me to get immediate natural feedback. If the tester gets stuck, let them sit with the problem and allow them time to figure it out. Only when they come to you for help should you nudge them in the right direction. If they get stuck for too long then your test has uncovered an important problem.

Any feedback is good feedback. However, good feedback does not have to be relevant feedback. As the developer of the game you still get to choose what feedback you want to listen to, and what you won't. A tester does not always share your vision, and sometimes a tester may even be from outside the target audience. You may find it helpful, before a test, to write down what it is you want feedback on. Then, as you evaluate the feedback you got, categorize it either as strictly relevant, loosely relevant, or a new idea. This will allow testers to give any and all feedback they have, while not completely changing the scope of your project.