How should one deal with someone who says "Alhamdulillah" if it is feared that they might not be Muslim?

The questioner is advised to ward off thoughts of disbelief from himself, for the general principle is that we judge people by their outward actions, and leave their inner thoughts to Allah, as the Messenger of Allah, peace and blessings be upon him, said: "Indeed, I have not been commanded to delve into people's hearts or to rip open their bellies" (Agreed upon). The Prophet, peace and blessings be upon him, also rebuked Usama ibn Zayd for killing a man who had uttered the declaration of faith, saying: "Did you not split open his heart?" A Muslim is commanded to act according to outward appearances, and has no means of knowing what is in people's hearts. Scholars have unanimously agreed that worldly rulings are based on outward appearances, and Allah takes charge of inner thoughts.