If you're writing a program that outputs to a printer and want to be able to Peek into a location to detect whether the printer has issued a DSR signal, the info you seek is on page 54 of the SSC manual. Checking location $C0E9 will give the status of DSR as read from the ACIA status register. If using some non-Apple card, check that card's manual.
Article Change History:
16 Dec 1996 - Reviewed for technical accuracy, revised formatting.