There isn't really apart from 2 of them and Identing
The redundancy is in the way the FMC works to allow you to follow the pink string. Even then the SOP's I have worked to have said the NHP should have the primary nav data displayed on there side.
If I had a split between gps and VOR track I would check the following
1. I am pointing towards the point I want, not one in another country to the one I am in.
2. The beacon I am tuned to. Has the box done a double flip.
3. Status page to make sure it hasn't gone into DR mode. And one of us has cycled the messages to get rid of the annoying light flashing and not spotted the DR message amongst the other trivia.
If I have done all that I would then follow the VOR and presume that the point is wrong in the GPS database. Which has happened to me before.
Some how NEW got shifted west by 10 miles. If we had followed the GPS we would have busted Newcastles zone. The Captain didn't really like GPS and went with the track towards NEW then at 60M to run turn 15degs to make sure we passed to the east of the zone method. It proberly won't happen again, but if it does I hope I remember the lesson I learned that day.