you said spelling mistakes, grammatical mistakes are good?

The context is different here. What I meant to say is that, in general people will make mistakes and professional writers and bloggers whose primary job is writing tend to make lesser mistakes.

So to identify a unique scenario of someone employing a paid content writer can be identified quickly by the lack of spelling mistakes. ie one quick metric but not very accurate.

Also this is based on my personal habit of missing out spell checks and never doing grammar checks.

my last question would be at what % should a post be declared as plagiarized, because I had been recently caught by cheetah for using a sentence

I am not sure of how @cheetah works. For example some software has identified this very post at 18% plagiarized. but the content is in quotes - ie copied content. So the percentage is subjective. This article for example is not plagiarized but it definitely resembles many & the final 2 paragraphs have ripoffs from popular movie disclaimers and "don't be fooled by randomness" is from a book by Nassim Nicholas Taleb