How do muslims determine who’s following Islam correctly and who’s not?

Muslims who follow different sects and madhhabs argue amongst themselves and don’t always share the same islamic beliefs or follow the same teachings. One moment, you’re arguing with someone who believes the qu’ran is the direct word of Allah and follows the hadith as well, then you argue with someone who only follows the qu’ran and then you suddenly come across someone who believes the qu’ran has been corrupted but also still believes Mohammed split the moon. How do muslims determine who amongst them is following the ‘correct’ way of islam? What happens to those who ‘don’t’ even though they believe they do?