The front flex disk is checked by torquing the driveshaft back and forth with the trans in Park (first or reverse on a manual), parking brake off, and looking for cracks. If the disk rubber is cracked, the cracks will be obvious.

If your drivetrain feels like it's made out of rubber bands, the front flex disk is suspect, and they usually only last about 60K miles. They're basically a maintenance item.

The rear flex disk is much more robust, lasts MUCH longer, and is easily checked the same way with the trans in neutral and the parking brake on. When checking flex disks one should always verify that the bolts are tight, but I don't think a rust stain has any signficance.

Duke