[ars]

That would be tough since we've never actually observed an event horizon. We don't know if they even exist.

The only observations we have are of supermassive objects. Do they have event horizons? We assume so, but no one really knows.

[sohkamyung]

We may get an answer to that in the future via the Event Horizon Telescope [1] which plans to "directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon."

[1] http://eventhorizontelescope.org/index.html