When you buy fish--any fish--always look at the eyes and then gently touch the skin. If the eyes look bright and the skin feels supple, then chances are it's fresh. This is if you go to the market as opposed to getting it literally off the boat.
If, on the other hand, the eyes look glazed and dead (well, yeah, the thing's ALREADY dead, but 'old' dead as opposed to 'new' dead) and the skin feels a tiny bit mushy, then it ain't fresh.
As for preparation of sashimi, most of the time over here it's just done with a little vinegar, salt, and sugar, and then dipped in soy sauce. Most Japanese chefs look askance at the American version of preparing sushi i.e. avocado rolls. To me, nothing wrong with it, but the chefs here are traditionalists for the most part. Good or bad? You decide. I prefer it the traditional way, and I prefer sushi to sashimi, as I like the taste of rice with a tiny dab of wasabi on it, just for the tang it gives. You don't need much, and less is often more.
With regard to the 'falsity' of sushi in N. America, it's always been mislabeled. Often they'll use pollock as the main ingredient in making fake crab, as it's cheaper and has the same consistency as the real thing. I tried sushi with my family the last time we went to TO, about five years ago. Meh...
Thread: sushi fans?