A non-Christian can appear righteous on the outside by doing a righteous act now and then but cannot be truly righteous because full righteousness comes only from Christ.
Who measures that righteousness
God. That's why the Holy Spirit (who is God) within a person will indicate whether or not one is on the right track which answers your third question.
If God is the only one that can indicate whether or not one is on the right track, then how is it that you know someone cannot be truly righteousness? Wouldn’t God be the only one that could know that?