Documentaries, googling a lot of things (and reading from reputable sources), looking at nutrition labels.
But taking a college nutrition class is what expanded my knowledge to where I needed it to be ngl I already knew a lot of the material from personally obsessing over it for years but that class taught me how to calculate calories accurately and I’m SO thankful
(labels will straight up LIE and pretend fiber cancels out carbs and list a lower calorie count!! almost bought these giant “90-calorie” tortilla wraps once..... calculated the cals off the label and they were really 120-140 cals per )
this freaked me out so much omfg