The Quran

وَلَو نَشاءُ لَأَرَيناكَهُم فَلَعَرَفتَهُم بِسيماهُم ۚ وَلَتَعرِفَنَّهُم في لَحنِ القَولِ ۚ وَاللَّهُ يَعلَمُ أَعمالَكُم

Muhammad Asad

Now had We so willed, We could have shown them clearly to thee, so that thou wouldst know them for sure as by a visible mark:¹ but [even so,] thou wilt most certainly recognize them by the tone of their voice.² And God knows all that you do, [O men;]

Lit., "by their marks": implying, elliptically, that God does not grant to anyone a clear insight, as by a visible mark, into another human being's heart or mind.
Lit., "the tone (lahn) of speech": indicating that a true believer recognizes hypocrisy even without a "visible mark" (sima).