fishstix

You are correct in your theory.

If in doubt find your demarc on the side of the house and open it up (demarc = demarcation point for the phone company). It usually is labeled "phone company" or something of the like. You should see your feed cables coming in and tied down to a block. I would take a look at it now and then look at it later. That should shed some insight as to what pair the installer used to feed the house.

I can also add - sometimes the cable that runs to all your jacks get wired in series. Somebody opens a jack up and forgets to tie down "line 2 wires" and the circuit is broken. If you happen to know the first jack from the cabling outside - I would start with that one and move throughout the house.