How do you know who is an infidel?

You believe that philosophical/intellectual recognition of the Creator is a measure of non-heresy.
Someone else believes that an infidel is someone who believes that not all of the Torah was given from heaven, but only parts.
The third says that emotion is necessary to worship God, basing his words on explicit verses in the Torah.
Rambam wrote 13 main points in total, and others less.
Today, there is also talk that someone who rejects Kabbalah is a heretic. Some believe the opposite – someone who believes in Kabbalah is a heretic.
 
Since there are no “laws for heretics,” and on the other hand it is problematic to accept the words of rabbis/sages as having authority other than formal authority,
How do we know who is essentially an infidel?