It is possible to tell a KO gun from the real one but only in person. I know because I had the chance to compare both as originally I had been sold a KO gun from ebay which I thought looked 'off' but then at the Parra fair, I bought another gun and compared them both. The KO gun is much glossier than the real gun from Jamie's table which was among several legit Constructicons.
Unfortunately this is impossible to tell in photos.
If anyone asks, I gave away the KO gun a long time ago.