These all succeed:

is_stream(user_input).
is_stream(user_output).
is_stream(user_error).
is_stream(current_input).
is_stream(current_output).

As does this:

?- open("/etc/motd",read,Stream),
   is_stream(Stream),
   atomic(Stream),
   blob(Stream,T),
   close(Stream),
   blob(Stream,TT).
Stream = <stream>(0x29db6b0),
T = TT, TT = stream.