Compare the same grammatical position in active and passive voice.
PassiveActivePosition
لَا يُفْعَلْلَا يَفْعُلْsingular masculine Ghaa'ib
لَا تُفْعَلْلَا تَفْعِلْsingular masculine Haadhir
لَا أُفْعَلْلَا أَفْعِلْMutakallim singular
First identify the Seegah, then use the internal vowel pattern to identify active or passive voice.