This is an ongoing question for small business owners...and one that is often difficult to answer. Our best piece of advice for small business owners is to ask yourself one question, "what am I missing?" and then make a list. If the following answers are on your list, you should consider speaking with a marketing consultant.
Defined brand and value proposition
Buyer Personas or customer segmentation
Lead gen strategy
This article from CIO will give you more ideas to consider when considering marketing consulting services.