Suspecting an undiagnosed learning disability in a student
I might meet with the student and ask about class performance without pushing any particular hypothesis. Going with the sandwich (good thing/bad thing/good thing) approach: "StudentName, I appreciate your active participation in class -- it's incredibly heartening -- but I'm noticing that your performance on assignments and exams doesn't show the same level of understanding that I hear from you in class. I really want you to do well here. What's going on? Can I help?"

And then listen. Have several referrals ready beforehand -- disability office, yes, but also the writing center, tutoring center, any helpful affinity groups the student might be appropriate for, and so on -- so that you can make the most useful, least overwhelming suggestions.