What is your "eureka" moment in the game when you know that you will lose or win?

I read a lot from this sub and Im wandering how do you guys know that you did the right thing and you will win? Or think "damn, I should have done this and that 20 turns earlier" now Im gonna lose. Im sorry, english is not my first language and I dont know did I made myself clear. Example: By the cca 80 turn i take 3 capitals and think, thats it Im gonna snowball from now on just to discover another continent and there is somebody waaaay ahead of you? Ps. Can somebody recommend me any youtubers , I mean there are so many guys providing tips for game that Im not sure who to listen ( one says he never builds granary or monument, the other one says he rushes them...) Ty anyways!