I am pretty sure I have leaky gut syndrome; how do I diagnose this?

Leaky gut syndrome. You could have a special test for this using lactulose and mannose. You could also have an IgG test for a panel if foods, and if you react to quite a few of them, you can assume that you do have "leaky gut" or increased permeability of the gut. In my practice we usually just do the IgG panel. .
See below. Leaky gut syndrome is not an established medical diagnosis. If your have gastrointestinal issues i recommend being evaluated by your primary md or gastroenterologist.