Typically this kind of problem is simply addressed with a physical device (a VOM, for example) attached to the pins in question.
in reply to Need to detect continuity between 2 terminals
Now that I have had some coffee (and reread the original question thrice) -- you are not asking to detect a physical short circuit between two pins on the serial/parallel cable, right? If you are asking 'How Do I Detect a Data-Ready condition on the line?', then the answer is 'it depends'.
Basically you will end up polling the connection (either in your code or in the OS), waiting for the next buffer of data from the line. We will need a little more information to start making educated guesses as to a solution.
I Go Back to Sleep, Now.