Be open, but be prepared that they may not be.
Mental illness is not easily assumed, but there are definite red flags that you may have noticed that have concerned you for your friend. Don't make a statement about those red flags when you approach the conversation. Always be an ear.
An ear listens instead of talking, and responds instead of rebutting.
Start off with an empathy statement that asks them how they are doing.
"Hey, are you feeling okay lately? I know this time of year always gets me down."
Ask open-ended questions without making assumptions.
"How have your classes been going?" instead of "You have so many classes right now, that's so overwhelming!?"
Respond without interrupting.
"I hear you.", "I understand.", "I would feel the same.", "That makes sense."
If your friend confides in you, offer your support and guidance. Never assume they want you to reach out to help on their behalf. Just ask. If they aren't ready, be a friend and not an enforcer. Healing happens at an individual pace.