The problem with Left for Zeds comment is he uses an obvious champion ability counters champion ability to explain a bad match up. It's not that simple all across the board nor does it answer the question.
Most games are won by Mechanics and Decision Making. LUCK is not a viable variable as often times "Luck" is just you or someone else's failure in one of these 2 areas.
Take Vayne and Caitlyn for instance. This is a matchup most players who aren't D1+ completely dread. Caitlyn's range over Vayne is incredible, and It's not as simple as "oh i'll use my tumble and close the gap so she's in my auto range!" By the time you've done that you've been hit 3 times and your 1 auto trade is completely not worth, and your escape ability is gone.
However imagine this. Say Caitlyn is around a wall. As vayne, landing your condemn is a high priority. Let's say you DECIDE(Decision making) to go in. You eat her 3 autos, DODGE(Mechanics) her piltover and condemn her into the wall. You get 3 attacks and proc your silver and then back out. You will have won the trade. How do you know you will win the trade? What Zed said, EXPERIENCE.
How do you know if you're "good?" Ask yourself things like this.
- Do my game decisions usually lead to victory? (Decision Making)
- Do I frequently win lane? (Decision Making & Mechanics)
- Do I frequently snowball my lead (or anothers) to help achieve victory? (Decision Making)
- What* role* do i play in a Teamfight as X champion, did i accomplish that role? (Decision Making & Mechanics)
- Did I miss a skillshot? ( Mechanics)
- Did I* dodge* a game changing skillshot?(Mechanics)
Now to answer your question. How do you find a champion you're good with? Take me for example. I REALLY love Quinn. And i like Vayne(but not like i love quinn)
Here are my answers with Quinn.
1,2,3,4,6 are YES. 5 is NO
Here are my answers with Vayne.
1,2,4,5 are YES. 3,6 are NO
There is just something about Quinn that makes me a better player mechanically and decision wise. I win more games with her, I enjoy the games i play with her more, I talk about her so much I'd die if she was real and i met her in real life. However, i loved her before i was good with her, before I knew what she was truly capable of.
TL;DR: Loving your character like a soulmate, and playing the shit out of them. Also Mechanics and Decision Making.