Print a single walled square (turn on allow single extrusion walls in the advance tab) that has a wall thinness the same as your extrusion width. that is of a known height. If the extrusion multiplier is correct the wall will be exactly the thickness when measured with a accurate caliper of your extrusion width.
Also can check starting height with same square as the height of the wall should be exactly what the model height is.
Here are some squares of different thicknesses, now sure what size you need, or make you own. They are all 2mm tall http://misc.airscapesart.com/cal_squares/index.html