Well there are a couple ways. Take a look at your perimeter layers as they go down. The edges should touch one another. If they dont them you need to up the multiplier so more plastic is extruded. You cant do this on first layer. Another way is to print a calibraton cube. Measure with calipers when done. If its undersize then increase the multipler. If its over then decrease. There are many calibration items you can download on thingiverse.