When buying tuna to eat raw, it must be “sushi grade”, which means that they are frozen and stored at a low temperature once caught, in order to kill bacteria.
To test the freshness of the fish follow your nose — if it smells fishy, don’t buy it. It should have a neutral aroma. The color should be beautifully opaque and even — tuna ranges from blush pink to deep red, depending on the species. Touch it — it should be firm yet yielding; if it’s tough, walk away.