Try using them to measure something you know the diameter of, like another seatpost or the quill of a stem. The accuracy of the calipers may not be perfect, but then again the diameter of something may not exactly be as quoted either.
+/- 0.2mm for a seatpost shouldn't really make much differance. I could squeeze a 27.2mm post in my 27.0 Zasker without worrying too much.