How can I tell if frozen or delivered pizza has gone bad?

A frozen pizza may develop ice crystals or exhibit signs of freezer burn. Ice crystals break down the cellular structure of toppings such as onions, mushrooms, and meat, causing them to release excess moisture as the pizza bakes. Signs of freezer burn include dark, leathery patches. A baked  pizza's dough or toppings may exhibit mold growth. The pizza also may impart an off flavor or odor--both signs of spoilage. Toppings such as meat, onions, and mushrooms may appear slimy--an additional sign of spoilage.
 
Source(s):
VirginiaTech "Food Storage Guidelines for Consumers"
Healthgoods.com "Food Storage Guidelines for Consumers"