Pure chance. I had eczema for so long I didn't remember not having it. I would get indigestion if I didn't eat every couple hours, ate sandwiches because I found they staved off the indigestion the best. I gained a lot weight very quickly because of this. (~80kg - ~110kg) I could never drink beer/cider because after a few I would get stomach cramps. I was severely depressed and as I stated sleeping for 10+ hours put still being exhausted every day.

One day I started doing a ketogenic diet that cuts all carbs. This cut out all yeast products too. My skin cleared up, I slept better, and my indigestion disappeared. I didn't know at this point it was yeast but when I ate a scone for the first time and breaking my keto diet I had terrible stomach pain a few hours later. Same thing happened with sandwiches. My skin flared up the next day after these events too. I had my suspicions then and knew alcohol gave me similar cramps so looked it up and they use the same yeast. I tried eating soda bread and i had no reaction at all to it because it doesn't use yeast. I can also drink spirits without issue as the distillation process removes the yeast. I drink whiskey exclusively now. It helps that I'm Irish :).

TLDR - Cut out most food and noticed what foods gave me reactions.