if you pay attention to the scales on pinks or any salmon that is darkening for that matter. the lower the scales are to the belly the fresher the fish. salmons scales slowly dissolve into their skin as they mature (happens fastest with pinks) but most the scales above the lateral line will stay intact so you want to take a quick look or feel for scales closest to the white of the belly, now if the belly is turning red,orange,grey and even black (depending on species) then these should go back no question because they are past prime.
another way you can check is to give the fish a little squeeze with a couple fingers along their backs and see how tough the meat feels. i find this not as good though as most salmon still feel tough even when pretty dark however this is a good way to check pinks as they soften up so quickly.
I normally only keep pinks in the ocean or tidal sections of rivers when they are bullets and not showing signs of maturing at all. every male i have kept this year hasnt even had a hump
or even really any signs of a hump.