I don't think any reasonable person could honestly believe there is any significant possibility[0] it could have been faked. The Soviet Union would have had a vested interest in proving it was a hoax if it were, yet they uttered not a peep of protest.

Faking it and keeping that information from getting out, especially over this time span, would be a ludicrously wide-ranging and leakproof conspiracy that continues to this day.

[0] Because really, anything is possible. We could all be a Boltzmann Brain that came into being mere instants ago, that's just a useless thing to invest any serious consideration into.