If I want to hire an agency of marketing then how should I know that these guys are best in that work? and not scam people!.
Could you explain what you mean by marketing before I attempt to answer this question.
You can do some research on them. With some common sense and researching skills you can weed out the bad ones.
As eoinfinnegan said, the word marketing can mean many different types of promotional activities, so you need to be specific about what kind of marketing you are talking about.
you can easily to find them with the help of feedback rating as well as service