They do a test call Edinburgh post natal depression test that would determine if you were, it's very easy to get it confused with the baby blues pretty much all women get this its only if it's really effecting your life or it lasts a long time that there's cause for concern. X
For example for about the first couple hours or so after you give birth it is common for you to not have much interest in your baby. Of course you may seem excited immediately following bit women usually go through a period where they want to talk about themselves and their accomplishment of childbirth and may request the baby be in the nursery or may not seem interested in doing much with the baby (this is not saying every mom does this but it is the norm and is due to sudden fluctuation of hormones) however after that period if those behaviors persist mom will be closely monitored for other signs of post partum depression
You get a questionnaire after you give birth, if anything comes up odd they bring in someone to talk to you. Also when your nurses come in to check you throuought your stay they are also assessing your behavior for warning signs