If you have ANY suspicion that there is or may be abuse, please be a "Mama Bear" and keep a close watch. If anyone caring for her is less than forthcoming with information, that is a red flag. If she has been a "difficult" baby - colic, illnesses or disabilities - abuse can creep in when parents are frustrated and tired, so another thing to consider. If the living situation is difficult, parents not together, money problems, unstable housing or employment...

I do not suggest jumping right in with a report to Child Protective Services (CPS) because the family may cut off contact with you, and those hurts may never heal.

What I suggest instead is that you educate yourself on the signs and symptoms of abuse (these are not always marks on the baby - they can be attitudes, language and behavior on the part of her adults, or even older children in the household, or daycare if they are involved.) Then cast an eagle eye on everything. Take every opportunity to be the one who bathes, feeds and diapers her. See how she reacts to people - fear or withdrawal can be more signs something is amiss.

Can you have an calm, open conversation with one or both parents about what happened? Remember, even if it is an accident, they will be feeling a great deal of guilt or remorse, so you need to sort that from actual abuse.
I am sorry you are in this difficult position, but please stay alert and do what your heart tells you is best for this little one.
I'll bee thinking of you.
Sue