For the same reason a German speaker can tell if someone is faking that they can speak German.

For the same reason that a molecular biologist can tell if someone is faking that they've taken organic chemistry.

When someone has studied multiple religious traditions not for the purpose of heresy hunting, but rather in the spirit of the Golden Rule, the consilience of their common teachings can be distilled.

It requires dropping the familial ego bullshit, i.e. "MY family taught ME all the right teachings." I dropped that decades ago.

It also takes a lot of proof of work. Adherents to a single religious tradition lack that proof of work.