I think what you're asking is: how do we know whether the question is addressed (formally) at a single person or at a group. There is no formal/informal distinction when talking to a group, you always use vous.

The distinction between formal 2nd person singular and 2nd person plural is only possible through context.

The question itself is not formal or informal.  With whom you are speaking determines which phrase you will use.  Informal is used with people well known to you and not in positions of authority.  Formal is used when speaking to "strangers" or your boss.