“Ist dieser Platz noch frei?”

Word for word: “Is this place still free?”

/ɪst ˈdiːzɐ plats nɔx fʁaɪ̯/

A shorter, very common version is 'Ist hier noch frei?', and in beer gardens and canteens it's normal to ask it and then share a table with strangers. On long-distance trains, check the small display above the seat: it shows whether, and for which part of the route, the seat is reserved.