LPT: If you suspect child abuse, do not wait until you believe you have evidence of abuse to report it to Child Protective Services, that is their job. And, very unfortunately, they probably won't be able to use your evidence anyway.
I found this comment (link removed, LPT doesn't allow links to Reddit comments) and I thought I would share it as a LPT. Please go ahead and read/upvote the comment, but just in case I'll paste it here as well.
I work for CPS and I can say this is the language we use. "No reasonable explaination" is used pretty often in our investigations.
Edit: Since this post has over 500 upvotes, I'm going to throw out when to report abuse.
DO NOT wait until you believe you have evidence of abuse. That is OUR job, and, very unfortunately, I probably won't be able to use your evidence anyway.
Call your state's child abuse hotline when you notice any bruising on a child that looks suspicious. Bruises that are linear, hand shaped, or on the fleshy parts of a child (i.e. their forearms, stomach, thighs, neck, or their cheeks. Bruises on boney areas are generally caused by bumps).
Call us if a child has constant lice or bug bites.
Call us if a child relates there is no food in the home.
Call us if a child is acting out sexually.
Call us if a child relates that they are homeless.
Call us if a child relates their parents hit them on the head, with a fist, or choke them.
Call us if you have reason to believe there is domestic violence or drug use in the home.
Call us if a child states they feel unsafe in their home.
If you wait until you can provide me evidence, it may lead a child to be in an unsafe situation. The above list does not include every reason you should call Child Services. If you ever find yourself questioning if a child is unsafe, it's our job to find out. If you feel as though a child's life is immediate danger, always call the police and they will contact us.
Our goal is not to break up families, or anything like that. It's to provide families with every service available in order to succeed. You can report anonymously.
EDIT: Some commenters are saying that as a child they had CPS called on their parents when there was no abuse, and it was an overall bad experience. I am not involved with CPS, nor do I know very much about how they act, but I do believe that they're doing the best they can. Hopefully they will be able improve the way they carry out their investigations as to have the experience be more pleasant for everyone involved.