You don't always know.

Recently, I saw someone say online that women suffering from PPD know it. That's a myth. I speak from experience that sometimes you don't realize that's what's going on. One of the things that can come along with PPD is a feeling of emotional and mental numbness. I had that. I didn't care about anything and I didn't feel much. When I did feel anything it was usually sadness, anger, or some other negative emotion. If I had realized earlier on that I was dealing with PPD, I would have gone to a doctor sooner instead of waiting until it reached a crisis point requiring an ER visit and hospitalization. That's part of why it's so important for everyone to know the warning signs and be educated about PPD. If you know someone who has recently had a baby, keep a close eye on them. Know the warning signs of Post Partum Depression.

From http://helpguide.org/mental/postpartum_depression.htm

Lack of interest in your baby Negative feelings towards your baby Worrying about hurting your baby Lack of concern for yourself Loss of pleasure

Lack of energy and motivation Feelings of worthlessness and guilt Changes in appetite or weight Sleeping more or less than usual Recurrent thoughts of death or suicide

Postpartum depression usually sets in soon after childbirth and develops gradually over a period of several months. But postpartum depression can also come on suddenly, and in some women, the first signs don’t appear until months after they’ve given birth. Because of the possibility of delayed onset, if you have a depressive episode within six months of having a baby, postpartum depression should be considered.

If someone you know is not acting like their normal self and you think it could be PPD, talk to them. If you have to, talk to another family member of theirs. Do what you need to do to get them help before it gets too bad.