If you start asking questions like: “How do you know you love me?” , “How do you know we are soulmates?”, “Isn’t it still early for all of that?” , “Aren’t you moving a bit too fast?”, he will become defensive and he won’t like any of those questions.
He might even get annoyed or angry because he is not used to this kind of situation. He is not used to someone questioning his words and actions, and this is a whole other dimension for him.
This is probably the only situation in which he won’t have all the right answers. And these are the questions you should ask to see his reaction.