I have seldom seen a post where I agree with so much conflicting advice at the same time iyswim
On the one hand, there are horrendous examples of children being abused and nobody reacting. We all know that this happens. This may be a genuine, desperate cry for help, in which case this child needs you.
On the other hand, children also tell lies. I knew someone whose sister made up stories about being mugged by older children on the way to school. The Mum got into trouble with the school for pointing out that it couldn't physically have happened and that the child often told lies- it made her look like a dodgy parent, because the school stuck very firmly to the line that the child should always be believed.
Dh had his Mum storm in to his primary school to complain that the children were being taught wrestling- (she still feels embarrassed about it over 40 years later). He just wanted his school day to sound a bit more interesting.
Dd used to bite her arm and then say little brother had done it (you can tell by the angle of the teeth marks).
My nephew was taken to clinic after complaining of unbearable earache; when the doctor failed to find anything, he admitted quite cheerfully that "I was only joking". He was a happy healthy child, just too immature to realise that there was anything wrong with this type of joke.
So both possibilities must be born in mind, both scenarios can and do happen.
On the one hand, the advice not to question this child yourself is wise- you could end up putting ideas into her mind.
On the other hand, trusting that all social workers etc will have the experience to accept that a child may lie also seems a little blue-eyed- as we all know, there have been cases where professionals have believed almost anything. Remember the Cleveland case?
I would do what an earlier poster suggested and ring an advice line anonymously. Explain how it is, that you have not actually seen any abnormal bruises, that you are aware that it may be a lie, but that you are worried by the possibility of it being true and ask where they think you should take it from there.