I believe you are using the term leaky gut incorrectly. Leaky gut is when holes are created in the gut (by candida) and you become allergic to nearly everything you eat. You get what I call the “leaky gut burn” when food particles enter the bloodstream through the holes that are in your mucousal membranes and your body creates an immune response. Other “reaction” symptoms include headaches, nausea, hyperventilation, ringing ears, irritability, etc.
Basically it feels like indian rug burn but throughout your whole body. People who get leaky gut have typically been constipated for a very long time and have had candida for years.
In order to heal leaky gut, you need to be on this special diet for a long time, and you need to take specific supplements that heal the gut. I was able to heal mine within a year or so and I had it very bad.
To test whether you still have leaky gut or not, you simply eat foods not allowed on the diet such as grains, sugar, etc. and see if you get the allergic response I mentioned above. You can also get specific allergy tests for a proper diagnosis…you will be allergic to nearly everything (I never did this).