The senses test - smell, look, touch and taste. If it fails any of those, or rather if it doesn't pass all of those, chuck it. Freeze it, or cook it, or cook it in a dish (like pasta) and then freeze it.
I like to make salmon tartare in a large quantity when cheap, and freeze it in small portions. Serving it just before it is fully defrosted in summer is divine.
Too many restaurants, not enough time...