The generic function stream-listen is called to determine if there is data immediately available on the stream stream, without hanging.
result should be true if here is input, and nil otherwise (including at end of file).
This method must be implemented for subclasses of buffered-stream that handle input.
There is a built-in primary method specialized on buffered-stream which returns nil. There is a built-in :around method specialized on buffered-stream which checks for input in the buffer and calls the next method if the buffer is empty. Thus a primary method specialized on a subclass of buffered-stream need only check the underlying data source.
The built-in method on fundamental-input-stream uses stream-read-char-no-hang and stream-unread-char. Most streams should define their own method as this is usually trivial and more efficient than the method provided.
LispWorks User Guide and Reference Manual - 20 Sep 2017