How did Hank find out about Walt?

Related questions

1. Will Jesse be in better call Saul?
2. Should I watch Breaking Bad before Better call Saul?
3. Does Jesse die on Breaking Bad?
4. Why did Breaking Bad end so soon?
5. Will watching Better call Saul ruin Breaking Bad?
6. Is breaking bad any good?
7. Will Walter be in better call Saul?
8. Why does breaking bad has 62 episodes?
9. Does Netflix have better call Saul?
10. Why did huell take the ricin cigarette?

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22. #perusing
23. #season