How one may recognize whatever fish fillet is fresh?
The usual answer is smell, but sometimes it is not possible to smell the fish if already packed.

If the whole fish is fresh, ask the fishmonger to filet the fish for you, or do it yourself.

It is not that complicated; there are tons of videos showing you how to do it; and is usually cheaper than buying pre-packed filets (except if buying filet from a large fish, it might not be practical!)

If you have only access to packed filets, check for packing dates, it should be, IMO, no more than 1 day.

If there are no packing dates, ask the people working there, if you are not certain do not buy it.

If you buy from a supermarket chain (I do sometimes), check to see if there is a high turnover of products; do people buy the fish, and do the employees re-stock the shelves on a regular basis.