eHow offers tips on Sushi and how you can tell if its fresh:
1. Observe the restaurant for overall cleanliness and orderliness, and notice whether it has a pleasing aroma when you walk in.
2. Look at the fish displayed in the sushi bar before you order.
3. Observe the color of the fish. The fish should not have uneven coloring or many dark patches.
4. While eating the sushi, notice the texture. The fish should be soft and melt in your mouth. Exceptions are octopus and squid, which should be wonderfully chewy.
More tips here.