How can you tell if a shellfish is still alive?

To determine if a shellfish is still alive, you can tap on its shell; if it closes tightly in response, it's a sign that it is alive. Additionally, check for any movement or opening of the shell. Fresh shellfish should also have a clean, ocean-like smell; a foul odor indicates it may be dead or spoiled. Always discard any shellfish that remain open or do not respond to touch.