Both are of your sentences are possible in AmE as questions about whether someone is part of a queue, but "Are you in line?" would be far more probable.
You can use 'the line' to refer to a specific line/queue or a certain kind of line/queue. For example:
- Why do I always seem to end up in the slowest line?
- Somewhere in the line behind me, someone started complaining loudly.
Student or Learner