is this how i should do it?

var lastposition : (whatsthedatatypeforthis?);

var transform.position.y = 0; if (lastposition != transform.position.y){ Debug.log = "moved" }

If you just want a test on the height, it's a float then. Something like :

var lastHeight : float; // renamed to be more coherent

function Update()
{
// It's better to do it that way, float comparison can give you surprises due to lack of precision.
if( Mathf.Absolute( lastHeight - transform.position.y ) > 0.1F )
{}// Here the height changed, to whatever you need to
lastHeight = transform.position.y;
}