The only way to tackle this, I think, is to keep a detailed food diary. If you have issues after having visited a particular chain restaurant, then next time you visit that particular chain order something very different - different food cooked in a different way - and a different drink.
Hopefully after a while you'll spot a pattern. Perhaps, as you say, it's what they cook the food in (e.g. the oil or whatever) which is the issue. Perhaps it's a particular ingredient. Only a food diary will tell you that.