Any thread in the process that owns the queue can use DosPeekQueue to examine the elements in the queue to determine which one to actually read. Each call to DosPeekQueue returns the identifier of the next element in the queue, so the function can be called repeatedly to move through the queue. The identifier of the desired element can then be supplied to DosReadQueue.
Any process that has opened a queue can use DosQueryQueue to determine the number of elements in the queue. This function also returns an error value if the queue owner has closed the queue.