The words given by "learner" are all good. It can also depend on what country you are from and who you are speaking to. For example, at a ticket booth you could ask "Which seats are vacant in this section?" You could ask a person sitting in that row the same question but it comes across as a bit too formal and stilted. If you are at the actual seating area and speaking to a fellow customer you could ask them, "Is this seat available?/ Are these seats available?" You could also use the word "free" such as "Is this seat free?" You could use the word unoccupied as well here but , again, this would not really be used speaking to a fellow customer as it would be a bit too formal . To a ticket vendor you could say, "Which seats are unoccupied?" when speaking about a particular section though you would usually ask "Are these seats occupied?" if you are interested in a particular set of seats or section.

You can use these adjectives to describe an empty seat which is not being used: free; unoccupied; vacant