Well this is a steaming pile of brown stuff. Mohammad Yousuf is probably one of the top Test batsmen in the world and Pakistan have a policy of banning ICL players from the national side.
That said, the way the Pakistan Cricket Board found out is quite amusing. They phoned his house and asked to speak to him. His wife said he was out. The PCB asked where he was. She said he’d just left for India to play in the ICL.