You could compare the magnitude of the velocity of the previous frame with the magnitude of the velocity of the current frame (or rather, the sqrMagnitude, which is faster):

private var oldVelocity : float;
var crashThreshold = 10.0;

function Start () {
    oldVelocity = rigidbody.velocity.sqrMagnitude;
    crashThreshold *= crashThreshold;   // So it works with sqrMagnitude
}

function FixedUpdate () {
    if (oldVelocity - rigidbody.velocity.sqrMagnitude > crashThreshold) {
        // Crash
    }
    oldVelocity = rigidbody.velocity.sqrMagnitude;
}