What he said ^^^^^^
I used to get this every now and again from one supplier when they got to the bottom of the freezer and tried to palm me off with the ones that were either refreezes owing to thawing during delivery or just left too long in the freezer.
Sometimes you can tell by the look of the animal but generally the smell is the main clue - Taking back one stinking rat and enquiring if they would feed that to anything they cared about seemed to make the point.
dr del