When God is in union with you, he reveals many things to you, leaving you with no doubt whatsoever. For example, I just know that the Lord exists and that my own existence depends entirely upon him. If you ask me how I can be sure, I can't say that he spoke to me aurally and produce a recording of his voice for you to hear or anything like that – all I can say is that he has simply deposited the knowledge in my heart. If you still have questions, you just have to seek union with God and find out for yourself. Know that I can tell you about God, but only he can reveal himself to you.