I always suggest going to your local machinist's supply store and buy a "reference standard" 1 inch long. That will be a piece of steel precisely ground to 1.0000 inches and tracable back to the National Institute of Standards and Technology. It's an excellent piece to keep handy to check a caliper or anything else from time to time to be sure it's still accurate.
Close the caliper and the reading should be zero. Then measure the reference standard and the caliper should measure 1.000 inches. This means your caliper is right on the money. You can also use this standard to test the pressure necessary to get an accurate measurement.