Then, once he begins to speak, his words disclose far more than any facial expression could ever betray. Concerning the hypocrites, the Exalted has said:
“If We so willed, We would show them to you and you would surely recognize them by their outward marks; yet you shall certainly know them from the tone of their speech.” (Muhammad 47:30)
Thus Allah made it clear that the hypocrites must inevitably be recognized through the nuance of their speech, while recognizing them by outward signs depends upon His will. The lying hypocrite utters with his tongue what his heart does not contain, and so the very inflection of his words exposes his falsehood.

About the believers He said:
“Their mark is on their faces, borne from the trace of prostration.” (Al-Fath 48:29)
And regarding the disbeliever He said:
“Harsh—and on top of that, ignoble.” (Al-Qalam 68:13)
That is, he carries a stigma of evil—a distinctive sign by which he is known.