We are in territory that I actually know a lot about. I managed a fish market in Southampton, NY for almost five years. I know raw fish. I have been to M. Slavin a couple times and was unimpressed overall. Like most places, one needs to judge each individual type of fish, there will always almost be both good and not so good in a case.
If you want whole fish it is easy, just look at the eyes. Clear and bright is what you want. The cloudier they are the older they are. Transluscence and firmness apply here as well (see next paragraph).
For filet, look for transluscent and firm, without any cracking in the filets. Ask to smell it and put your nose right up to it, you want no smell at all. You can even touch it, if it feels greasy, it is old. Filet can easily be "refreshed" with a saltwater or bleach solution, which removes most of the smell and greasiness, but it can still crack and loses some transluscence.
If you are interested in sushi grade tuna, touch and taste. A good fish market should let you taste it. If you touch the tuna (in this case you want it to be greasy from a higher fat content) it should be a little greasy (we can go into grades in more depth later if there is demand) and firm, not separating. Taste it! You can also ask to see a whole filet to check the bloodline, but if you need to do that it probably is not worth eating.
In all the fish that have blood lines, they should be red, bright red. Tuna, swordfish, mahi mahi...I am always amazed at the grey or brown or dark blood lines in fish that people purchase.
Well, that is all I can think of at the moment. I buy quite a lot of fish at Super H Mart in Falls Church. It is almost all whole fish and you choose the fish and select how you want it chopped up or fileted and they do it on the spot at no charge. They also have head on shrimp which is outstanding and denotes that it has never been frozen, but make sure the antennae are long and the head are firmly attached and the guts are not rotting through the shell. Again, there should be very little smell on the shrimp.