A common sign of tetralogy of Fallot on an X-ray is a “boot-shaped” heart, because the right ventricle is enlarged. Oxygen level measurement (pulse oximetry). This test uses a small sensor that can be placed on a finger or toe to measure the amount of oxygen in the blood.
https://www.mayoclinic.org/diseases-conditions/tetralogy-of-fallot/diagnosis-treatment/drc-20353482

