Checking the IIgs serial port to see if a character is waiting

[rtk <oneelkruns@gmail.com>]

I have some code I wrote for QForth many years ago that works with the
IIgs serial port.  It is based on code from ADTPro.

Presently, I have words in Forth for reading a character from the port
and writing a character to the port.  What I'm curious to know is if
there is a way to tell that there is a character in the port waiting
to be read but without blocking waiting for one as the read routine
does presently.  I'm hoping it is as simple as check the high bit on a
memory location or similar.

A pretty specific question!  I suspect there is, but someone with
deeper knowledge, and a better memory, than me will know.

Any help appreciated!

Ron