People of the qiblah are recognized through outward signs such as prayer toward the qiblah and profession of Islam. Affirmation of the essential teachings brought by the Messenger ﷺ lies at the foundation of faith.
Outward legal status and inner reality do not operate on the same level: religious rulings concern what is outwardly established, while Allah fully knows the hidden states of hearts.
One may not, on mere suspicion, claim certainty about an individual Muslim's inner state or final destiny. Evidence and the limits of the law must be respected.