No observational proof of the black-hole event-horizon
  Recently, several ways of obtaining observational proof of the existence of black-hole horizons have been proposed. We argue here that such proof is fundamentally impossible: observations can provide arguments, sometimes very strong ones, in favour of the existence of the event horizon, but they cannot prove it. This applies also to future observations, which will trace very accurately the details of the spacetime metric of a body suspected of being a black hole. 
Marek A. Abramowicz, Wlodek Kluzniak, Jean-Pierre Lasota