It's a very good question! In general, there is nothing regulatory that either makes something sushi-grade or not. We use our best judgement from being chefs to now being intimately connected to the seafood industry and also being a huge fan of sushi to determine whether or not something is sushi-grade. Some things help us make our decision.
- One might be how the fish was bled. If a fish is not bled properly, it won't be good for sushi. Not because it isn't fresh, but the blood imparts a flavor that is undesirable for raw seafood.
- Surprisingly, most of the fish used for sushi here is previously frozen for convenience. When served raw, the freezing doesn't affect the texture or flavor of the fish very much. However, we prefer tuna that hasn't been frozen.
- Generally, we will recommend something as sushi-grade if we personally know how long the fish has been out of water, how well it eats raw (some fish will never be sushi-grade, because it doesn't have a nice mouthfeel), and how it's been treated after it was caught.