For Cases that are in a Queue, what field in the Case document corresponds
to the Queue? If I were just looking at a Case and wanted to see in the
details what Queue it was in, I wouldn't know unless I went to the Queue to
see all the Cases.

You may have already figured out. When a Case is assigned
to a queue the case does not change ownership. Nor is
there any reference to the queue itself in the Case.

Queue/case info is found in the Queue-related tables and
you see cases that are assigned to queues thru the UI in
Workplace.