The #1 symptom is basically allergies to all/most foods. An allergist can determine whether you have allergies and whether they are more than the average person.
This is caused by inflammation of the gut.
Another common symptom is malabsorption of the food you eat. What you’d notice in your stools is food from previous meals coming out poorly digested. This can also be caused by not chewing your food enough.
I knew I had it because I suddenly became allergic to everything rather quickly. Eating an apple would give me an allergic reaction for hours for example.