How do you tell the difference between adjustment disorder and normal bereavement?

Impairment. Bereavement is a natural consequence of loss; it allows us to mourn that which we have lost and readjust our world view. Although it makes us sad and may disrupt sleep and concentration for a time, it has a pretty predictable course. Adjustment disorder is a reaction to some sort of situation that causes disturbances in mood and/or behavioral that causes problems for the person and others.
Maladaption. The hallmark of an adjustment d/o is the maladaption. This leads to depression, anxiety, behavior and conduct problems. Bereavement does not cause these problems to as severe of a degree.
Depends...? So, the rigid definition is...You don't. Basically, that's why we have psychiatrists. That is, doctors who are specifically trained to tell the difference between a problem, and a "not problem." or, so to speak, trouble, and a "normal life" event. The dsm is great...For research. But, human life is simply too subtle and nuanced to lend itself to any such neat and clean categories!