If you’re having difficulty cutting through fur/hair and it’s not coming off quickly, then chances are the blades need sharpening. If there is a lot of pulling or an excess amount of resistance when cutting, chances are your blades need sharpening.
One way to test if they’re dull is to slide the blade against another surface (table, arm, etc.) and see if it catches any material as you pull the blade across it. If it does not catch anything one way or the other, then chances are the blade is dull.