Control throws are measured usually from the trailing edge of the control surface's neutral point when centered along the trailing edge of the wing (ailerons) or horizontal stab (elevator) or vertical stab (rudder) to the trailing edge point when deflected for both high and low rate settings. Most instruction manuals will give you both the recommended throws (in inches or millimeters usually) along with a visual diagram.
When the control surface is deflected, measure using a ruler or measuring tape to locate the approximate recommended throw. Place the starting point of your measuring tape or ruler at the trailing edge's neutral point and hold it there while moving your control surface to deflect in one direction. Measure that distance holding the control surface in the deflected position. Adjust your throws accordingly (less or more depending on your initial measurement).
Many advanced pilots can eye ball control throws but it is highly recommended for those newer to the hobby to measure your throws. One day, you will be able to "eye ball" the throws and then dial them in after the maiden but until then it is good practice to take the time to measure.
For a more precise method of measuring, there are many hobby measuring tools online. Try searching for "control throw measuring tool for rc airplanes" or similar.