How can you tell something is alive?

What do you have to know about an object to be able to determine if it is alive?

Depends

Well, if you can check it's breathing. Not all living things do though, just check. Check if it's moving, try giving it food and see what happens.

Dead or Alive

In mammals the only certain way is signs of RYTHMIC breathing.

You can also check for reaction to noise and eye reflexes.