Before every game, I first check the setup I’m using to make sure neither is missing any cards.
I do this not by ordering them ace through king, but by sorting them by rank (the four aces together, the four 2s, four threes, etc.).
While doing this I am looking at the faces for signs of fading, scuffing, dings, edgemarks or other wear.
I then flip them over and do the same for the backs.
It is important to have good lighting and also to view the cards not just straight on but from an angle, since some dings and marks are only seen obliquely.
Any card which can be identified by marks, fading or other wear I replace from a backup setup.
The checking process takes me maybe 6-8 minutes total for two decks.
I mark the box for one setup as the replacement deck from which spare cards are taken. If you’re lucky, the same card won’t get messed up for a long time so the two setups last longer. But eventually you’ll either need a third setup or two switch to some different cards entirely.
If the cards are all worn to an equal degree (e.g. fading of ink on the back) so that you can’t use it to identify cards, they can still be used; but I do like to replace them once noticeably worn.
Since moving away from cheaper plastics, fading and other overall wear is much less and much slower. I used to use plastic Hoyles, which were cheap but got easily dinged and also faded quickly on the backs. This was a problem for replacing cards, since the ink on a replacement would look much fresher than the rest of the scuffed pack and be easily spotted by anyone looking for it.
The two Faded Spade (3.0, four color, poker sized) setups which I’m currently using have been used in about 7 games so far. I’ve had to discard only two cards due to dings, and there is zero scuffing/fading. The dings were on random low/middling cards and probably came from someone aggressively peeling back their hand with a card protector on them. Unfortunately the FS 3.0s are sold out, and they seem to be about to issue new cards, so I don’t have a third setup to continue if necessary…