Bloomberg does this with data sales. They'll sell you particular financial information, like tick-by-tick for the Russell 2000 over the last 5 years. They embed unique hidden markers or fake trades with everyone they sell them to. They then go and acquire data from other "providers", and find the leak.
From what I understand, they used to change volume slightly which they claim wouldn't negatively impact algorithms looking at historical data. Then, however, people were using volume weighted average pricing (VWAP), which would eliminate that change. This is just theory now, but I believe they just add double trades, instead of fake trades are certain prices.
