Observe the crab shell, weigh it, and smell it.

1. Observe the crab shell: You can observe the moisture and looseness of the crab shell. The shell of fresh crab is tight and moist, while the shell of dead crab is dry and loose.

2. Weighing: The weight of crabs changes. The weight of newly dead crabs will not change, but the weight of crabs that have been dead for a period of time will become lighter.

3. Smell: Smell to determine how long the crab has been dead: Newly dead crabs have an earthy smell, and they will emit a rancid smell after being dead for a while.