Is a brain stem stroke diagnosable by mri, or is it just a clinical diagnosis? i have an inkling by looking at the symptoms that i had one.

Typically yes: yes typically acute stroke will show up on MRI brain. In very rare instances strokes can be not seen (especially posterior circulation related) and the term "MRI negative stroke" is used in such a scenario. If you think you had a stroke you need to go to ED immediately to get MRI brain and close observation as some strokes can result in a dangerous brain swelling especially in young adults.