How do google get know about clicks fraud

Many of u would be thinking that you can click the ads yourself by using different IP address or different location or different proxy server

sorry to say this trick of yours don't work

because

you get 5 visitors, and all 5 click the ads - your ctr (click through rate) is 100% that's nearly impossible without cheating, it raises flags.

If you get 5 visitors, and only 1 clicks the ads - that's 1/5 20% CTR - which again is suspicious and raises flags. But if you get 10,000 visitors and say 500 people click it - that's 5% and that's more realistic and less likely to set off false flags. Don't try to cheat Google because you'll be kicked out, they're really not that stupid.