While nobody could corroborate Ellie's personal experiences, anybody could prove the signal was impossible to fake. The simple one word reason is parallax.
Let's say you try to fake a signal from Vega. That's 26 light years away, so close enough to detect parallax easily.
You can put a satellite in orbit around Earth and make your satellite transmit a signal, and pretend the signal is from Vega. A radio dish could pick up the signal when your satellite is in the same position in the sky as Vega. All a radio astronomer has to do is ask a technician at a different radio telescope a few thousand miles away to point their dish at Vega. When the other dish points at Vega, it won't be pointing at your satellite, so it won't get the fake message from your satellite.
It's a simple parallax test.
Radio astronomers do this all the time. Every day. They might get some transient signal. The signal is not necessarily an artificial signal, but something from either a nearby source on Earth, something in orbit around Earth, some automated unmanned aircraft flying over their dish's controlled airspace, or perhaps a weird natural phenomenon hundreds of light years away. How do they filter out Earth based (or Earth orbiting) signals from astronomical events? They just ask their buddies at a different radio telescope to do a quick check. This check only takes a few minutes. If their buddy a few thousand miles away gets the same signal, they know the source is not from Earth. If their buddy does not get the signal, they know the source is Earth based.
I'm surprised the main character, Eleanor Arroway, did not mention this very simple test at the end of the movie when National Security Director, Michael Kitz, asked if Hadden could have faked the signal.
I am also surprised that anybody doubted the authenticity of the signal, or that Kitz even mentioned the possibility that the signal could be fake. The consensus by astronomers all over the world within the first few hours would be "this is real!"
While a good movie maker sometimes wants to leave doubt in the minds of the viewers that alternative interpretations are possible, a good scientist would say that no doubt is possible on some questions because the evidence is conclusive.
How do I know this? I've been an astronomer. I published peer-reviewed articles in astronomical science journals. There is a reason why my profile picture shows a radio dish.
There are 3 independent lines of evidence to support her story, so I think that people would eventually come around to believing her.
- The signal is real because you can't fake parallax.
- The recording devices contain 18 hours of static evidence, which supports her claim of being gone for many hours.
- Once her computers dig deep enough into the digits of Pi, the computers will eventually find "evidence" of a message hidden in Pi by the "creator" of the universe. This would show she was given knowledge which was not previously known to humanity, which anybody with a computer could independently confirm.