すみません、この席は予約席ですか？ — Excuse me, is this seat a reserved seat? (Public · Transport, Formal — to staff, JLPT N5)

Gesture & etiquette

Point to the specific seat.

Avoid sitting in seats with "Priority Seat" (優先席 - yūsen-seki) signs unless it's very crowded and no one else needs them, or if you yourself have a need. If unsure, always ask.