Whether you are buying from marketing agencies or freelancers the first thing to do is to check their reputation online. If there is nothing available to check then ask them for examples of their work.
In a perfect world word-of-mouth would be the best way to find a marketing agency/freelancer but this is not always possible.
It is very simple, look at their reputation online, look at the work they have done and make your own mind up. You could even use websites such as Upwork to find a long-term freelancer. There are options out there, don't be afraid to use them.