It is decided by bigger part of the society.
For example if you say "real man do not run away from problems, but face them" you will get agreement from majority of society.
If you say "real man should wax his legs" you will get agreement from minority of society.
Thus making the first phrase "true".
But there is no actual list because there is nothing that 100% of society would agree on.
It is fluid list. You say something, think if majority of society would agree and that is "true" to what real man does
That is how it works.