You could try an isolation diet. Just pick one basic food, such as steak or baked potatoes only, with no toppings except for maybe salt. And eat just that thing for a week or so. Then see how you feel after your gut adjusts. Then gradually add things back into your diet or start with a different food from square one. Not the easiest plan to implement by any stretch, but it's very effective as you're limiting confounding variables for figuring it out.