You'll need to know the methods your preferred SEO consultant will use to rank your site. This is important since there are bad SEO practices "black hat SEO" that can get your site banned on search engines. Common practices like cloaking which website owners could get away with in the past can now get your site banned, yet they are a bit too technical for the average website owner to understand.
So, the digital marketing consultant you hire must be able to discuss their winning methods clearly (do’s and don'ts) as well as offer an estimated timeline on when you should start expecting results. The SEO consultant's proposal should include a free review of your site to identify problems. The review should include a free report. He/she should tell you the "on page" optimization techniques they will use to solve your site problems (i.e. involving the site URL, internal linking structure, web page titles, tags, headings, etc.).
Most importantly, they must adhere to Google’s latest webmaster best practices which prohibit the most common SEO tricks. Yahoo and Bing also have a list of webmaster best practices that your preferred consultant must follow.