At least in the North Atlantic when reporting a position on the frequency, you also have to give next waypoint with an estimate for it and the following as well, so it's relatively easy to pick up when you are off track.
I'm not sure about the data sent via datalink, ADS-C and ADS-B though, but with ADS you can monitor if an aircraft is off track.
One more thing to consider, many routes are created based on winds at altitude, and those winds change over the day/year.