Can you tell us more?
Yes-again I have absolutely no doubt that the cannon pictured is legit, would bet anyone money 'cause I'd win.
What I did was to look at the marks. The maker is one that isn't listed in any books, none, and I've only seen two other cannons by that maker, and they are "out of the way" so I'm sure I'm one of two or three people in the world who would know those marks, especially the proper syntax and spelling of the maker's name. That marking would thus be impossible for a faker to fake since there's no available source of that particular mark, or there wasn't until they posted this cannon on the web. That's really the main clue I have to the authenticity. Plus the weight marks, who would make those up? The bronze has a seawater-etched look to me.
The fake cannons I've seen always have a relatively clean look inside the bore, even thought the look isn't correct for an original. The bore on this gun is full of carp (or maybe some other species) due to submersion.
Hope that helps. I've seen I don't know, maybe tens of thousands of cannons, and the fakes usually scream at me. This one says "I'm real."
Again, the fact that it is real and a relatively unknown maker and design is helping the crooks keep selling this same, already-sold-and-delivered cannon to probably countless other victims.