Excuse me, is this seat free?
li-BE-ro — stress on second syllable. Short, polite question.
When looking for a seat on a regional train where seating is open (no reservations).
'È libero' (is it free/vacant) is essential in Italian for seats, tables, and parking spaces. 'Questo posto' (this seat) clarifies what you're asking about. On regional Italian trains seating is open — arriving first means you choose any seat.
Questo posto è occupato?
Is this seat taken?
Alternative phrasing — same meaning
C'è qualcuno qui?
Is there anyone here?
Even simpler — asking if someone is sitting here
Posso sedermi qui?
Can I sit here?
Asking permission rather than checking availability
On Italian regional trains (Regionali), seating is open — no reservations required. On Intercity and high-speed trains (Frecciarossa, Italo), all seats are assigned on the ticket. Even if a reserved seat appears empty, the passenger may board at a later stop.