* you do not experience intestinal discomfort on a regular basis
* you can eat a wide variety of foods without getting unexpected diarrhea or constipation (i.e. when eating foods that typically do not do this to a wide range of people)
* your bowel movements are consistent, complete, a good colour, not painful, not loose, not hard
* you are not overweight/underweight
And of course, you can help all of this along...
* basically never eat fast food if you can avoid it, especially shitty McD's ground beef
* avoid taking antibiotics unless you're really sick enough to need them they can wipe out your microbiome and set you back months or years of progress)
take supplemental calcium and vitamin D
* eat a wide variety of foods, including oily and spicy things; you can "train" your system to handle things by eating them in smaller quantities as you get started and then gradually eat more
* but avoid eating too wide a variety of foods in one meal as that's most likely to overwhelm the gut's capability to effectively digest things
* yogurt, cheese, fermented stuff of almost any sort (without alcohol) all seem to be beneficial
* while we're at it, make sure you get enough iron, from red meat and dark green veg!
It's anecdotal of course but you can find any number of studies supporting this stuff. For me, taking my diet seriously didn't mean restricting in any serious sense how _much_ I eat, or even _what_, so much as balancing things to ensure I don't overwhelm the system.
In 2017 I had to take antibiotics for something and it seriously set me back years, I was back to having a horrible time at least once or twice a week from things I had been eating no problem before, and only by late 2019 was I really feeling confident to go and eat, say, spicy Mexican food again. At this point in time I would have to be sick enough that I'd be missing months of work to take antibiotics again.