You can't really tell by looking at it.

No, there’s no visual trick.

“Sushi-grade” isn’t a regulated term—it’s a marketing label meaning the fish was frozen to a temperature and duration that kills parasites. Most grocery store tuna has been frozen at some point anyway. Your best bet is to check the label for “previously frozen” or ask the fishmonger directly. If the tuna is labeled “sashimi” or “sushi-grade” and looks bright, firm, and doesn’t smell fishy, you’re probably safe. But don’t rely on color alone: oxidation, not quality, turns tuna from deep red to brown.

When in doubt, freeze it yourself for a week at 0°F before eating raw.