A rotten eggplant will have some discoloration, be bitter and have soft skin. The shelf life of an eggplant is very short.
Eggplants should have firm skin. Any wrinkles or soft spots is a sign of the eggplant beginning to decay. When pressed, the skin should quickly go back to its original shape. Bruising or discoloration is another sign of rotting. The skin will begin forming brown or tan spots. The stem should not have any mold present. An eggplant is only good for a few days, so buy it right before cooking. Cook a small piece to test the flavor; if it is bitter, then it is already beginning to rot.