How to Hunt for Hidden Photons - Technology Review: "There doesn't seem much prospect of ever seeing hidden photons but there is a way we might see their effects because they ought to interact very weakly with ordinary photons. And by the probabilistic laws of quantum mechanics, ordinary photons must sometimes become hidden ones and vice versa, oscillating back and forth, albeit very rarely.
Experiments are already afoot to spot this kind of oscillation. The idea is to shine a powerful laser at a wall with a photon detector on the other side. Since ordinary photons cannot pass through walls, any that do pass through must have turned briefly into hidden photons and back again."