2. Look at your mouth. Sometimes, I touch its head with my hand, and when I see no response, I just throw it away. However, this does not prove that it must be a dead crab. At this time, you can wet its mouth with water (don't soak it in water). After a while, see if it will spit bubbles. If there is, it proves to be a live crab.
3. Look at the crab legs. If you have tried the first two steps, you still can't. You can turn it upside down, let its belly face up, and gently dig its navel (the female crab is a round navel, and the male crab is a pointed navel) to see if its legs or body have any signs of activity. If it is active, it proves to be a live crab.