Is this seat taken?

Is anyone sitting here?

These are the two most common ways that English speakers ask someone if a seat is available (in the cafeteria, on a bus, etc.)

Also, in this context, “take a seat” or “take your seats” doesn’t mean to move or steal a chair; instead, it is a polite way to ask people to sit down (usually at a public lecture or in a classroom), as in “Everyone please take a seat.” or “Please take your seats.”