discernment of truth and falsehood

عَفَا ٱللَّهُ عَنكَ لِمَ أَذِنتَ لَهُمْ حَتَّىٰ يَتَبَيَّنَ لَكَ ٱلَّذِينَ صَدَقُوا۟ وَتَعْلَمَ ٱلْكَـٰذِبِينَ

Allāh has pardoned you, [O Muḥammad, but] why did you give them permission [to remain behind]? [You should not have] until it was evident to you who were truthful and you knew [who were] the liars.