This has always been a closely guarded secret. There is no official resource, so far as I know.
From Cat Murdock
You can filter your champs in game by the star rating and at the very bottom there is an "unowned" selection and it will show all the ones you don't have for that rarity. Then, you have to use some narrowing down to those you know aren't in the basic pool (Titan locked, content locked, money locked, etc.)
This has basically been my method, and I look at the previews of champions in the crystal library to figure out who's in the basic (i.e. right now it shows Jameson first thing, so I assume everyone released earlier than him is in the crystal.) There are always just a few champs that trip me up; for instance, Solvarch (has he been added yet or is that happening this or next month? Same will go for Imperiosa down the road. Also champs like OG Vision and Scarlet Witch - I know OG Scarlet Witch finally became a basic 6-star but did Vision as well? Has he been a basic 6-star for a long time and I never noticed? It's just questions like these that would make it awesome if we could sort champs by crytsals as well and not just rarity. Ironically, I care less about the 7-star basic pool because there are always so many resources/posts about it - I'm just trying to fill out my lower rarity collection lol
It's crazy how mystic went from the weakest class when I left to the most busted class when I returned, I remember people begging kabam to give the mystic class some love 😅
