Default Route

How would I go about monitoring if my default route changes and being alerted?

Perhaps there is a different way of going about this.  any suggestions would be great.

Here is the scenario:

Traffic between Site A and Site B normally flows through a WAN circuit.  This is layer 2 so it connects directly to a switch at both sites.

Given a failure of that WAN circuit, traffic will automatically be redirected to use a VPN tunnel that is established between Site A and Site B.  This of course utilizes the internet connects at both sites.

Thanks,

Daniele