The first sign that frozen food has deteriorated is the colour of the food will have changed. The more dramatic the colour change, the more deteriorated the food is. Also, if the surface of the food is dry and is covered with pale grey spots, it likely has freezer burn. Freezer burn occurs when moisture on the top of the food evaporates. To prevent freezer burn, store food in zip lock bags, freezer bags or in non permeable plastic wrap or containers, removing as much moisture and air as possible before freezing.
Frost on food indicates slow freezing or that food has partially thawed and refrozen.
Do you know how to tell whether frozen food in your freezer has deteriorated, or how to prevent freezer burn? What this video to learn some facts and tips about freezing and freezer burn.