An image forming system includes: a reading unit configured to read a test image, the test image having a determination image formed by overlapping a plurality of colors and a plurality of reference images formed with each of a plurality of reference colors; and a determination unit configured to determine a first color value of each of the plurality of reference colors based on a reading result of the reference images, determine a second color value of a plurality of pixels included in the determination image based on a reading result of the determination image, and determine whether an image defect has occurred based on an angle of a second vector corresponding to the second color value of the plurality of pixels with respect to a first vector corresponding to the first color value of each of the plurality of reference colors.