There are a few simple tests. Look at your belly - the area to the left or right of your belly button. Try to hold that area between your thumb and index finger. Can you hold it? You can not, if you have belly fat. The measure of a flat stomach is thickness of the two layers of flesh between your fingers should not be more than an inch. Stand straight and hang a masons' plumb rule, holding its line touching the middle of your chest and its bob reaching just above your feet. If the bob hangs freely without its line touching your belly, it is the indication of a flat stomach.
There is a much simpler test. When you take bath, look at yourself in the mirror. Are you happy with what you see? If you have a bulging stomach, you will definitely feel awkward and promise yourself to do something about it. But the problem is, once you get nicely dressed, you will forget about it and go back to your usual ways of bad living habits. Your dress hides most of your body defects. Some people believe that sleeping immediately after lunch causes bulging tummy. It is not true.