It's entirely subjective, like wine tasting. Sweetness in such a bitter drink is more about what flavor notes exist that your brain associates with sweetness. Apple spice coffee, cinnamon, nutmeg, and certain floral notes like vanilla will pull "sweet-adjacent" associations in your mind that form a melody with the taste and smell.

I'm no expert and definitely don't have the most refined palate, but from what I've heard and experienced, comparative tasting is the best way to start to dig into and understand flavor.

For me, sweetness is a big one here. Since coffee isn't as sweet as a lot of other stuff we eat, having other coffees to benchmark against with more or less sweetness will help make it more noticeable.

its not ever directly sweet per se but when its balanced and possibly a bit juicy (not overly astringent, bitter, etc) then that can be described as ‘sweet’

I agree with the comments here. Personally, the sweetest coffee I have tasted is natural processed Kaffa Anderacha, which managed to keep some sort of cherry flavor no matter which roast level. I think you might get a real feel for sweetness in coffee with a quality natural processed coffee from Africa. The only issue here is that it was pretty tart, or sour\acidic, which, to my understanding can be alleviated by having it a little darker roasted, but that might also degrade the cherry taste.