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Lay on your back and bend the legs so your feet are flat on the floor, have the palms on the floor arms at the sides. Slowly raise the torso and slide the hands along the floor, don't jerk or use momentum or anchor the feet. If you can come up, it means your using the core muscles, jerking yourself up to do sit-ups is using the back. Doing 50 sit-ups jerking the body up isn't doing much for you, slow controlled sit-ups with unanchored feet will work your core which will stabilize the spine.

I'll try what you recommend when I finish the 12 lesson course. I used to practice "crunches" instead of sit-ups since this type of exercise does not cause me any back discomfort whatsoever. I did them the way it is recommended in the "10 Steps to a Better Body" booklet.

You can do the test anytime, if you can't do it without lifting your feet, then it means your not using your core and relying on jerking or the back.