Implantation is (generally) 2-6 days after O. That's why it is not worth testing at anything less than 6dpo..because baby is unlikely to have implanted. Some people can tell that they have implanted by cramps or by the loss of a small amount of blood. Other people, like me, have no idea when implantation has taken place!!
Wow, I'm getting to be a pseudo-expert on the 2ww 
Alice