If you prepare raw sushi at home (there are many types of cooked sushi, and entirely vegetarian sushi as well) be sure to use flash-frozen sushi-grade fish. It's more expensive, but the flash-freezing kills any (at least most) parasites. I always inquire about the quality and freshness of the fish any and every time I get sushi from a restaurant. Try this link:
http://www.sushifaq.com/ffaq.htm
Hope this helps!
Hamachi is usually a good metric for freshness, as the flavor and character of it degrades in a very quick and predictable way with time. I use it as a benchmark for the likely freshness of other fish for which it is not see easy to discern the age and quality of it. Of course, one has to be pretty familiar with hamachi first to do this effectively...