There are tests (lactulose manitoll test) but they may not available in all countries. Easiest is to make a 36 hour fast followed by a radical change to the diet and see if there is improvement.
How do i know if i have a leaky gut? And how long time does it take to stop breakouts from a leaky gut? I mean if having a leaky gut causes food allergies and you just consume a lot of that your gonna have acne anyway? Must be an easier way?
When offending foods (the one that cross the leaky intestinal barrier and trigger inflammation like gluten, lactose) is removed improvement can be seen in 3 - 10 days. To heal the leaky gut it takes much longer and (3-6 months) and when you have successfully done that you can again eat gluten or lactose with no ill effect.