They should have the mark down in the accident book and be able to account for it. That much you can expect of a nursery.
But I have to say both my children at that age and later would regularly blame others both for accidents that had happened when the supposed aggressor was not there and for injuries they had inflicted on themselves (I got used to checking the angle of bite marks). I don't know why, it just seemed to be a phase they went through.
I know that some people say that you should always believe the child- but I would have had to believe that 6mo baby brother had been writing the letters of the alphabet on the carpet with a felt tip pen and that a relative had been nipping over from Sweden in the lunch hour to bite somebody. There were other times when I really could not know who had hit whom or done what.
Sorry, this doesn't help you at all, because you're still left with that nagging fear. Because sometimes children do tell the truth.