To do this you will need an assistant.
It doesn’t matter which leg is done first as both legs will be tested.
Lie on your stomach on the floor and have your assistant carefully raise your thigh by lifting your bent leg vertically to enable a pillow or rolled up towel to be placed under the thigh just above the knee. DO NOT ASSIST by trying to raise the thigh yourself. Once that has been done have the assistant draw the heel slowly and gently toward the buttock until a firm resistance is reached ensuring that your pain level does not reach or exceed 7 out of 10. Hold the stretch for 30 seconds and measure the distance from heel to buttock. Have the assistant return the leg to the starting position and repeat on the other leg.