Well, as I understand it (and I've not checked up on it recently) but you cannot be 100% sure with any post intercourse method of birth control that fertilisation has not occured. Of course different pills work in different ways but it's always been very very dodgy ground in my experience.
If I've misunderstood it then let me know. I might even look it up in the WT lib to see what it has to say on it.