You can add hidden field to the list which is always sets by workflow (and only by workflow). Then you will see if workflow called the event receiver.
Or
You can create HandleEventFiring class in your workflow project and use DisableAllEventFiring and EnableAllEventFiring before and after updates in workflow
public class HandleEventFiring : SPItemEventReceiver
{
public void DisableAllEventFiring()
{
this.DisableEventFiring();
}
public void EnableAllEventFiring()
{
this.EnableEventFiring();
}
}