Hiring a great SEO starts with understanding yourself. Just like having a business, if there is a problem because you do not educate yourself then it is entirely in .If you think you need to hire an SEO expert, learn some SEO yourself first. Read Beginner's Guide to SEO, learn about links and why they are important (and the difference between good links and bad), why is content for SEO, and may even sign up for something like DistilledU to take a crash course in a structured format.
Once you understand the basics of SEO, then discuss your project with several people. needs of every business is different, so do not be fooled into thinking that SEO is a one size fits all type of business.
As said above, SEO is not a one size fits all business. business and industry will determine how effective it will be, how long it takes to significantly affect your business, and how competitive the search results page (SERP) is the landscape. For example, if you are a local dentist you should try to get into the list of questions to your question (eg denver pediatric dentist) but if you are a market of international travel you will be competing against a lot of big brands such as Hotwire, Airbnb, and its kind.
Therefore, you need someone who understands your space and what is necessary for the competitive ranking. I would not recommend hiring someone who has been doing SEO for a direct competitor of you, just because the long term you will be better off with someone with fresh ideas. If you hire a person who your competitors are used, at best you will be playing copycat while they move and work with the next SEO expert.
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Instead, I suggest thinking a higher level of certain types of businesses (let's continue with the theme of the dentist). You should instead look for someone who has extensive experience performing SEO for local businesses. If you have some dental practice, then you should look for someone who has experience doing multi-site SEO for local business for law firms, car dealers, and the like.