How are Muslims to tell the difference between a true and a false prophet?

When a person claims they are seeing the angel Gabriel giving them a message from Allah, there are a number of possibilities: - The experience is real. - This person is sincere, but experiencing a mental illness. - This person is manipulating the gullibility of simple minded people that can't think for themselves. - Other possibilities How are Muslims to tell the difference between these possibilities, and therefore be guided towards a true prophet of Allah?