Like fruits, humans have been eating raw fish for millions of years. Fish can be much better consumed raw than meat, for containing far less connective tissue.
Consuming food containing sugars and fibers within 2 to 3 hours after having consumed raw fish or egg yolk can cause severe cramps. Try eating fruits before consuming sashimi, and take your time.
To judge fish on freshness, it has to be a whole fish ; the slime-layer must be intact, and the gills should be moist and bright red. (except for mackerel, whose gills are brown) The scales must be both intact and shiny, and firmly attached to the body. The cut-wound edges must have a fresh scent. The flesh has to be firm. Poking your finger in the flesh, it has to bounce back. Fresh fish has convex eyes, which shouldn't be bloody but bright white, with shiny black pupils. There should not be any signs of browning due to oxidation. Just remember, fresh fish does not smell.
Of course nobody will tell you (or they tell you it is needed to kill parasites), but be sure the fish hasn't been in the freezer. You can tell because fish that has been in the freezer is tasteless and 'watery'. Supermarket-salmon often isn't very fresh and sometimes 'slightly prepared', to increase shelf-life.
Of all fish, consuming raw red tuna is safest. Farm raised salmon is much safer than wild salmon. Consuming raw fish other than tuna, you just need to make sure not to consume any digestive organ of the fish, that you check the flesh of the fish, and that your immune system is working properly, so that your body is perfectly able to prevent any possible parasitic infection.
Fat fish can be consumed raw best. Red tuna can therefore much better be consumed raw than white tuna. Many fish can be consumed raw, like farm raised salmon perfectly can. If you want to consume raw mackerel, be sure it's very fresh (sashimi), because you might react allergic to not-so-fresh mackerel. (due to originated amines) Cod and herring very regularly contain much parasites, and should not be consumed raw. In most countries herring has to be deep frozen for 24 hours to kill possible herring-worms. So the raw herring they sell, is actually never ‘clean’-raw. Snapper contains very little parasites.
If you do not like playing Russian roulette, do not consume Blowfish (Fugu), Globe fish, Swell fish, Puffer fish or fish of the order Tetraodonitiformes. These fish contain a deadly poison (Tetrodotoxin) to prevent being consumed by larger fish (and humans).