How to know if someone pirated your game?

@Poryg @bgillisp
The idea of releasing pirated version yourself has been done several times before, you can search the internet for it.
It is one of the tycoon-type games, and about the player trying to control a game developer studio. I don't know the exact name and there are several out there, but the way it was done was very fitting:

In the pirated version, the developer added a realistic feature to the game: If your products (the pseudo-games) get successfull, they get "pirated" and your game-income from them drops. In the legit version there was no simulated software piracy.
And it got to the point that several people even went to the official forums to ask how to defeat the softwae piracy in the game, not realising that they out themselves as playing with a pirated version.

However there is one disadvantage to a copy protection that ruins the game: a lot of people will assume that this is a bug in your work, not a copy protection, and proclaim on the internet that you are a developer for buggy games.

A better way would be to turn the "pirated" version into a demo - let the player play through half the game before giving him a screen "thank you for playing the demo, please purchase the full version to continue.
A lot of pirates don't play the games they pirate, so they might not even realise this in hacked veraions.

@Andar : Right, that's why I did propose putting the demo on those sites, but with nothing stating it was a demo. Maybe I can manipulate the file length too so it matches the file length of the full game so there is no way to check it via that too?