I started by looking at screen-caps of the rifle from the only episode in which it was ever used, "Where No Man Has Gone Before". While it has quite a bit of screen time, it was never large enough or sharp enough in a film frame to see any possible "tells" or unusual details that could be used to match the photos from the auction.
So I moved on to something I've had great success with in the past – PR photos. The rifle was used in at least 2 rounds of PR photos, first when the pilot was shot, and later when the show was actually being launched. Low and behold, I was able to find a great shot that shows conclusively that scratches seen on the sale piece are also evident on the piece in the PR shot. The great thing about scratches is that they are irregular and organic and virtually impossible to fake. Judge for yourself in this photo which zeroes in on the front of the rifle's body:
I've numbered 3 areas that show an absolute match. The color difference is simply the lighting. I've found some other marks that match up, but nothing as clear and specific as this.
While this might seem like flimsy evidence, in prop collecting this is as good as a fingerprint. It's not IMPOSSIBLE to fake, but it's really, really, really hard. And when combined with the total accuracy of all the structural details, I think this makes it a 100% certainty that this is an original – THE original – Phaser Rifle.
So if you've got the better part of a hundred grand, bid with confidence!