When the false Messiah shows up in a few years to save the world from the global economic collapse that will take down all nations, he will declare himself to be God.
That’s when you know he’s not a good guy.
That is when people who have denied Yeshua as Messiah and Lord will say "here's the messiah...people told us he was not God but a man like us."
BTW, the anti-christ will not claim to be God, but exhalt himself as such.
From Daniel 11:
36 And HaMelech shall do according to his will; and he shall exalt himself, and magnify himself above every g-d, and shall speak monstous things against the G-d of g-ds, and shall prosper till the wrath be accomplished; for that is determined shall be done.(OJB)