There are several tests used to measure hip flexor flexibility. An athletic trainer, physical therapist, or orthopedic surgeon can test you.
One test is called the Thomas flexion test. You lie down on your back on a table with a firm surface. The crease of your buttock should be at the edge of the table. Bring both knees up to your chest. Keeping your back flat on the table, lower one leg until it is straight out. Lower that leg toward the tabletop as much as you can without arching your low back or letting your pelvic bone tilt.
A tight hip flexor muscle will keep you from lowering your leg all the way down to the table. A flexible person will be able to get to a horizontal (normal) or beyond horizontal position (hyperflexible). The physical therapist uses a tool called a goniometer to measure the hip angle during this test.
Another test is the Ober test used to measure flexibility of the iliotibial band (ITB). This band of fascial tissue comes down along the side of the leg from hip to knee. For this test, you lie on your side with the leg in question on top. The lower leg can be bent to help support you on the table.
The upper leg is bent 90 degrees at the knee. The therapist lifts the leg away from the body to a horizontal position and then extends it backwards slightly. The leg is then lowered toward the table until it starts to rotate or can't go any further.
A normal amount of motion allows the leg to be placed in the horizontal position. With a tight ITB, the leg stays up and won't drop down towards the table. The extra flexible person can touch the knee to the table.