I was unable to get to fish mongers, and so bought 2 sealed packs of cod from the chilled section in a local supermarket (each pack had 2 loins, a total of 280g per pack). They were not purchased frozen.
I can't find the actual picture, but the packaging looks very similar to this (I've removed the make/producer)
The use by date is today and when I removed them from the fridge, I noticed a lot of liquid despite them being "sealed".
When I opened the package, each cod loin was in liquid.
I'm not a professional chef, but my understanding is that anything sitting in liquid is basically breeding ground for bacteria (hence why the advice is to defrost under running water).
I can't smell anything wrong with the fish - there is no fishy smell and the loins are white as one would expect.
Is there any way to tell if the food is spoiled or not before eating it?
As an additional question - does the fact there was so much liquid in each 280g pack of 2 loins (I would guess each pack had around 3 table spoons of liquid) suggest the fish was frozen?