You'll know if the pistons are seized when you try to push them back into the caliper to install the new pads. You should be able to use a screwdriver between the old pad and the rotor to lever the pistons back into the caliper. You can do this one pad at a time before you remove the caliper from around the rotor. If you remove the caliper first you run the risk of one piston pushing out too far as you push the other piston in -- especially if the hose is still connected to the caliper and the brake line and the bleeder is closed.

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Tom Savage
Vienna, WV
1984 300D Euro
1995 S320