In my own opinion, of course...
Check the "smell"...it should be sort of "sweet", and definitely not "fishy"...
If buying whole fish, check the eyes...they should be very clear and unclouded, preferably sort of "bulging"...
If buying fillets or steaks, look to see the fish is not "mushy"...if it appears to be "flaking", its probably getting towards "mushy", and should be avoided...
Dark streaks in tuna meat are to be avoided...
Elf makes a very good point, in asking, up front, when it was caught, but also, is it "wild" or "farmed"? Wild is wonderfully better!
Also, buying "frozen" is not the worst idea of the week...Here in Central Ontario, whereas we have a transportation system that favours us (at least compared with what I anticipate you have to Colorado, with all respect!), we can get "wild" BC Sockeye salmon fillets frozen, that I have found most satisfactory!
From my BIL, a wonderful fisherman of NW Ontario, I learned to get a very sharp filleting knife (I use a Normark, now about 30 years old, but honed to a razor!) with a very flexible, thin pointed blade, and remove all skin, belly fat, and of course, every vestige of bones...
Fresh fish is, of course, a lot more tasty than frozen, the BIL is a bit of a "snob" on this point!...his family "must" eat the fish literally within 12-16 hours of being landed, or its only fit to be "smoked"...I get great results of "planking" my thawed frozen sockeye fillets on the BBQ...
Hope that this helps a bit!
Lifter