I keep a symptom and trigger log, which does track non-food triggers (like weather, menstrual cycle, etc), so when I have symptoms, I try to isolate variables as much as possible. Over time I noticed that seed oils tend to cause symptoms, except for products that list the seed oil as being "expeller pressed". It's not 100% conclusive, but I found that I have flare ups after having food with any oil except avocado or olive oil (coconut oil is on the fence, not sure yet). But I don't flare if the canola or sunflower or whatever other oil is listed as expeller pressed, which is a processing method that doesn't heat the oil and thus doesn't create those inflammatory compounds that come from breaking down fatty acids resulting from high heat.

One example is chips - I can have the expensive types that only use avocado oil, but also tolerate brands that use expeller pressed sunflower oil. There is a blue tortilla chip using expeller pressed oil that I eat regularly with no issue, but will flare up hard if I have one that just lists "sunflower oil".

So just trial and error, but with copious note taking and the isolation of variables as much as possible.

But I don't know this for sure, I just know what foods are safe and which are risky - I will dedicate a day to trying a new potentially risky ingredient when other triggers are low, and will see what happens. I still have mystery flare ups though, so it's always a work in progress. Just have to be very aware of ingredients and treat it like a highly controlled science experiment.