I am glad to see these dangerous counterfeits exposed more fully; I joined this list just to make these comments. They began appearing about 6 years or so ago, and have been sold in auctions of the most careful and reputable dealers. I bought one on eBay in 2005 or 2006, only to see an exact duplicate--down to strike and centering--less than a year later.
As mentioned, the most obvious characteristic is the lack of beads of the top of Christ's nimbus (I think that the original coin must have been pierced there, and the forger was unsure of his ability to re-engrave the beads properly). Though they appear to be of pure gold and are of correct weight, they are slightly thinner than originals and larger in diameter. The S in SIT is open at the top, and the first S in ISTE is connected to the rim by a die break. On the obverse, while the head of the doge is fully struck, that of St. Mark is not, with the left eye weak. There is a flaw above O in ANTO, making the ' look like a circle.
Finally, the appearance of silver strikings/pressure casts is conclusive proof that these are modern fakes, since no such "silver ducats" were ever issued by Venice--whose coins have been thoroughly cataloged for over a century.