Are any of these marketing companies that say they go operate run and design your Shopify store for you legitimate?
How does one know which ones are for real?
Some but not all. If they’re a commenter here on the forums, check to see their history and if they seem to know what they’re talking about. If they’re a company you found online, ask to see some of their work and ask about their past clients and projects. Unfortunately there are a lot of scammers out there, but just be aware and I think you will be ok 
There are many marketing companies that offer store management and development services on the Shopify platform. Some are legitimate and offer quality services, while others may be unreliable or even fraudulent.
Does anybody know of any that are legitimate and that can be trusted?