Did you ever get much from the 'real' answers here? Basically, saying the baby is crying is not enough information to analyze what is the cause. Some additional information is needed.
All babies are different and they don't come with their own instruction manual - which makes a hard parenting job harder. So the first thing to do is to ascertain the objectivity of the parent. That involves figuring out if the baby is crying all the time or if it seems like it's crying all the time. With me as a parent I would have given you a completely different answer 30 years ago than I would today.
Next, the baby - it can be assumed that babies do not cry for no reason. It is up to the parent, therefore, to observe pattern behavior (crying) so as to determine its cause. Food, diapers, sleep, lack of sleep, teething; crying is associated with all of these basic functions. They must first be observed and eliminated as being causal. If you are left with a baby that is truly crying all the time and there is doubt as to the cause then see a doctor. Seeing a doctor isn't a bad thing, being that I know of no first-time parent that didn't get overwhelmed with the possibilities of what could be 'wrong' with their child.