In all reality, you generally won't be able to tell if a strut is bad by looking at it. The plastic cover is just for added protection, and not all cars have them. Another note, bad struts don't cause vibration. They usually only cause thumping noises, and a ride that makes you feel like you are on a water bed. I will highly suggest that it is something else causing the vibration. How do your tires look? Look at the tread, are they chopped at all?