cookie isn't passing value

I just started a new web development job and one of my first "warm-up" exercises involves a cookie that's not passing the user name value into the login page. Since the code is at work, I can't reproduce it here, rather my intention is to get some "general principles" expertise. Problem is that I don't see anything obstensibly passed from a cookie into the method; so my question is, how to spot a cookie being passed in code or markup? What's a generic way of noticing a cookie being passed or received by a program? Thanks in advance.