Generally, only stroke will present with an acute onset, so it's important to observe how suddenly the weakness has occurred. Second, note the pattern of the weakness. As mentioned, paralysis due to stroke is sudden. Your perfectly healthy rat may be frolicking with cagemates one day and be stricken the next. Due to the anatomy of the nervous system, stroke commonly affects one side of the body, but not always.
Stroke is caused by the interruption of blood flow to the brain or spinal cord.
This may happen in one of two ways. First, a blood clot can plug vessels, starving nervous tissue, or a vessel can rupture causing bleeding. The extent of damage will depend upon the size and location of the area affected. Strokes to the motor areas of the brain will cause the one-sided weakness noted above. More ominous are strokes that affect the brainstem, which contains areas vital for life, such as heart and respiratory centers.