There are several ways to test this. First, stand sideways and look in the mirror at yourself. Do you have a large swayback? This is called lordosis and may be a sign of weak abdominal muscles.
Now lie down on the floor on your back with the legs extended and the arms behind the neck. Contract your abdominal muscles by trying to touch your belly button to the floor. Can you flatten your low back against the floor? If you can't, you may have weak abdominal muscles.
Now while keeping the abdominals tight and your back flat, lift your legs both up off the floor a few inches. Don't stay in this position if your back hurts or you can't keep your back flat on the floor.
If the abdominal muscles are strong, the back can be held flat on the floor. If the abdominal muscles are weak, the pelvis tilts forward as the legs are lifted. The back will hyperextend into lordosis.
Another way to check for abdominal strength is to clasp your hands behind your head. Don't pull on your neck. Keep your chin tucked. Now bring your head, shoulders and arms off the table as if doing a sit-up. Just come up far enough to lift the bottom part of your shoulder blades off the floor. If you can do this and come up equally on both sides at the same time, the muscles aren't weak.