If you have a rigidbody on your gameobject you can use the velocity value to determine if your player has died.

http://unity3d.com/support/documentation/ScriptReference/Rigidbody-velocity.html

if(rigidbody.velocity.y > 10){
print("player has died")
}

if you are using a Character controller you might be able to do the same, im not sure though :)

edit:

public float timeinair = 0;
public float deathtimer = 10;
private bool dead = false;

void update(){
     if(!player.isgrounded){
          timeinair += time.deltatime
     }
     if(timeinair >= deathtimer){
          dead = true;
     }
}

Hi,

I would build a simple collider below your platform, and when the player hit that collider, it means death. then you can easily define several similar colliders around your levels at different height.

This would prevent always checking in an update, that's not very efficient.

Bye,

Jean

well you could make it so that if your player falls to a certain point, then you basically respawn it to another.... Here is a script that i use:

function Update ()
{
    //check if you character fell off the platform
    if(transform.position.y < -10)
    {
        transform.position.y = 5.329594; //set the y axis that the player will spawn to
        transform.position.x = 5.203072; // set the x axis that the player will spawn to
        transform.position.z = -46.88659; // set the z axis that the player will spawn to
    }
}

where it says "< -10" it means that when the player is below that in the y axis, then it will respawn the player to where ever the next three transform.position axis are located.

attach this script to your player game object...

hope this helps

-Grady

(script from tornado twins)