Basically, make it a priority to go to high-end sushi restaurants sparingly to ensure quality, rather than going to lower- or mid-range options frequently. But, for those who don't have the money or the willpower, there are some things to look for in your rolls.
"I would definitely not order the tuna roll, first of all. But I do love eel. Eel is a cheap fish, it's probably not fake at most places, and it's obvious what it looks like." And nigiri rolls, or even sashimi, tend to be safer options because you can really see what is inside, and big hunks of fish are harder to mask with fakes. "The chopped-up spicy tuna -- that's crazy, you really have no idea what's in it." He also noted, in counter-intuitive fashion, that the cheaper a fish is, the less likely it is counterfeit. So don't be afraid to slum it, in sushi spots, at least.