First, here is a general list of activities you should concern yourself with if you notice them in your roommate, especially if they are done in excess:
Change in mood:
Is your roommate suddenly anxious or easily agitated after historically being much more mild-mannered? Do they seem sad or depressed? Are they going through rapid swings in mood?
Do you notice that your roommate is taking part in activities they didn’t before, like doing drugs, consuming alcohol, sleeping much later than usual, or not sleeping at all? In general, are they just doing things that are very risky to their well being? Are they becoming withdrawn and antisocial? Do you see that your roommate is staying home from class and/or work far too often? Are they indicating they have a constant negative, hopeless outlook on life? Is your roommate becoming violent, either physically or verbally? Do they make threats?
Change in appearance:
Is your roommate who was once very well put together now unclean or unkempt? Have they experienced rapid weight gain or weight loss? Are they always very tired?
We would never advise that you put yourself in harm’s way or potentially induce violence of any kind. But whether you and your roommate have become best friends over the course of your residency together or just casual acquaintances, you, at the very least, are probably someone who can make a sound judgment on their behavior and their condition. You might be the person who sees them more often than anyone else. So you might be the best person to help them out, whether through direct and sympathetic communication with them or through other means, like contacting friends and family on their behalf.