Blacksheep,
"Navigating with a secondary system is good practice and has been for centuries. There's not much effort involved in running a raw data cross check against an old fashioned paper map at regular intervals. How else can you be absolutely certain that your FMS is telling the truth?"
Exactly what we routinely do, that's how I knew it was accurate without GPS inputs. It's even easier when you have a moving map fed from the FMS, all that is required is to make a cross check that the displayed aircraft position ties in with a raw position from VOR/DME or NDBs.
The FMS even tells us when to change beacons on our Nav 1 box and which one it would prefer, although we do pre-empt it on most occasions because our routine checks keep us ahead of the game in that respect. We can go into various FMS info pages which tell us which satellites and ground based beacons it is using. It is usually working / computing from at at least half a dozen or so DMEs.
On this occasion, we could tell when the jamming ceased, because the aircraft symbol on the moving map twitched slightly, which probably related to about a hundred metres.
Obviously the concern is someone who might be placing too much reliance on GPS. If the whole system suffers jamming it immediately ceases to work at all.