Finding the door to a parallel universe

If there were a wormhole linking us to a parallel universe, how would we spot it? One suggestion is that it will give itself away by the curious way it bends light. This idea assumes the existence of so called "phantom matter" which has been proposed to explain how a wormhole might stay open. The phantom matter would cause light from another universe, passing through the wormhole, to diverge and emerge as a bright ring.