Or I can just post an answer.
The rear driveshaft has a rubber coupler, not a actual "flex disc" in terms of this topic.
To check the front flex disc (it's just called the "front" flex disc, even though it is the only one on the car), the car should be off it's wheels to allow the drive shaft to move. The drive shaft is then twisted back and forth and looseness is looked for between the front yoke of the driveshaft and drive yoke of the transmission. Essentially, you are looking at movement between the 3 bolts that hold the flex disc on to the transmission, and the 3 that hold it on to the driveshaft.
The flex disc absorbs torsional load shocks from the driveshaft and is considered a wear item. The rear coupler it's a good idea to keep an eye on, but it's really rare to see a failure of it. Since it's mostly rubber, it may be normal to see a slight amount of weather-checking or small cracks. I do remember replacing one once where it had larger cracks on the sides and had what looked like tire cords coming out of the cracks, some sort of reinforcement threads.
Gilly