The only way that I know of to determine the freshness of shrimp is by the shell and the vein. If the shrimp is fresh, its shell is brittle and easy to remove. As the shrimp ages, the shell becomes soft. For the vein, if the shrimp is fresh, the vein can be easily pulled out. As the shrimp ages, the vein starts to disintegrate making it more difficult to remove.