You can measure the root depth of each key bitting and lookup the measurement on the manufacturer specification sheets to determine the key code. The following table provides the bitting depth to key codes for the popular Kwikset key brand used for most home front door locks.

Using calipers, measure each root depth (red lines in the following picture) and compare the measurement to the Kwikset lookup table above. For this picture, red line #1 measured as 0.214” which equals a key code of 6. The complete key code for this key is 63323.