How do you know when a Champion needs buffs or nerfs?

Constantly I see posts that champion A needs a buff or champion B needs a nerf. How do you know?

Back in the day, I'm old, there was a game I played on the commodore 64 called Arkon I got good enough with the game to beat post people with a pawn. Did this mean that the pawn was OP? No it meant that I was good.

What I feel happens is that a person will play against someone who is better than them, and honestly there is always someone better than you. This person choses a champ and destroys the entire game. Was the champ OP or did you just play against a really awesome player?

Take Yoric. I bought this guy and went on a recking spree and got level 5. I was dominate whether I was top, support, or jungle. Then I went on this horrible losing streak where I was dominated worse than the gimp in pulp fiction. It got so bad I stopped playing yoric until I could figure it out because it was not fair to the team. Was I being matched consistently against better players or just players who know how to counter Yoric, or does Yoric have a really big problem late game?

Take the Tahm buffs on PBE. I play Tahm, and I was surprised to see the buffs because it felt good and balanced the few times I have played him. So does my limited data set mean he is fine as is or does it mean that I have been playing a slightly below par champ against bad players? Who knows because I do not have the thousands to possibly millions of data points that riot has on the champ. Riot can see if good players are losing to bad players when playing Tahm, while you cannot.

Bottom line is you either trust the company or you don't.