How do you PROVE infant abuse/neglect? They cant talk- etc?

Others are correct and as far as the physical abuse - the marks from abuse versus normal kid stuff are much easier to detect for doctors. A bump on the forehead, skinned knees, scratched elbows - all normal. Bruises on the thighs, back, shoulder, chest, face, patterns of cuts and bruises are less likely to be caused by the child's own behavior and the professionals know this.