Find the governor or syntactic relation first, then name the case.
دَخَلَتْ مُسْلِمَاتٌ. — What is the role of مُسْلِمَاتٌ?
Answer: nominative subject; ḍamma.
رَأَيْتُ مُسْلِمَاتٍ. — What is its role?
Answer: accusative direct object; kasra.
سَلَّمْتُ عَلَى مُسْلِمَاتٍ. — What is its role?
Answer: genitive after a preposition; kasra.