I think it's best to stay out of the picture and not get involved unless you actually see something going on that warrants a call. Social Services can be a scary thing for parents who are really trying the best they can. Some children can scream for long periods of time because of their age and disposition, and it isn't the parent's fault. Sometimes mental illness can cause this and make a parent too embarrassed to talk about it...you never know.

However....

I admit it does seem a little out of the ordinary. So this women said that everything is ok or did she have an explanation for what was going on...like, oh he doesn't want to take a nap, or he was being punished in his room because he hit his sister...?

You can also offer your help to babysit (or lend a hand with) the child to give her a ten minute break from the screaming which would get on anyone's nerves. This would enable you to get an inside peek to see what's really going on if she allowed you to help out a bit.

Depending on the relationship that the parent may have with the neighbour, I don't think parents are obligated to explain to their neighbours what is causing their toddler to have a crying fit. Toddlers cry inconsolably at times. My daughter will be two years in the next two weeks and we just moved into a new neighbourhood. I am convinced that the neighbours think that she is being abused, I would think the same thing if I were on the outside looking in. But if there is no reason to suspect abuse, keep out of it.

My toddler throws a tantrum and goes into a crying fit before bath time in the morning and before bedtime like clock work.... screaming and dramatic wails that would set any concerned neighbour into action... Furthermore, there is now a newborn in the house so my toddler is really acting up, some people may understand the dynamics of raising toddlers, others dont.

Crying is a natural behavior for a child and it has a lot of causes. Hearing it an imagining a cause is not the same as knowing what's going on with the child next door. I wouldn't call CPS based on crying but it's a very valid reason to pay attention to what's going on next door.

Also while children aren't easilly controlled you do have a right to some quiet. If the crying is constant or occurs at inappropriate hours with no sign that the mother is able to stop it don't feel that you have no right to do anything about it. File a complaint with the apparment manager or home owner's association.