وَعَلَامَةُ الْمُنْكَرِ فِي حَدِيثِ الْمُحَدِّثِ، إِذَا مَا عُرِضَتْ رِوَايَتُهُ لِلْحَدِيثِ عَلَى رِوَايَةِ غَيْرِهِ مِنْ أَهْلِ الْحِفْظِ وَالرِّضَا، خَالَفَتْ رِوَايَتُهُ رِوَايَتَهُمْ أَوْ لَمْ تَكَدْ تُوَافِقُهَا
Translation: When a transmitter’s version is compared with reports of reliable, proficient preservers, it opposes theirs or scarcely agrees with them; this is a sign of munkar in his reporting.
Muslim gives an investigative method, not a judgment based on a label alone. Compare relevant transmissions, study the nature and frequency of divergence, and then evaluate. “Scarcely agrees” points to an evident pattern, not every isolated wording variation.