I would google bunch of SEO related keywords (like "how to rank high on Google" or something like that).. see who pops up in the SERPs.
If SEO people/company can't SEO themselves.. that tells you a lot about them.
You don't take diet tips from overweight people. You don't take investment tips from homeless people.
Don't take SEO tips from people who don't SEO themselves to get business.
My 2 cents.
Step 1) Ask them "what terms are you ranked for?"
Step 2) Listen to them make excuses about why they are not ranked even though the call themselves SEO experts.
Step 3) Cross them off the list.
The SEO consultant you work with better show you they are good enough to rank themselves. Otherwise they are selling snake oil.
If it helps-- we are ranked for some of the most competitive terms out there. Feel free to shoot me a call request if you'd like to actually RANK your site instead of pour money into it for months only to find out google has penalized you.
I'm biased toward Dan's answer, being one of the SEO's in the Clarity network. ;-) However, I'm not currently taking SEO projects, so I can speak freely here from nearly 20 years of experience in the field.
The answer to your question has many layers, and can easily head you in the direction of a snake-oil salesman without some additional clarity.
As one example: Why SEO? Often the term SEO is used by "buyers" to mean something much broader than the "seller"/technicians view of "search engine optimization", creating an opportunity for things to go sideways from the very beginning. For instance, business owners frequently came to us seeking better visibility in search engines with an end goal of attracting more businesses - though it was fairly common to field requests for the technique du jour (SEO, PPC, Social Media) vs. the end goal. The thing is, the most cost-effective route to achieve the end goal might may not be the technique du jour. Propose the end goal, and let the experts map out a way for you to get there with the best ROI.
Jami's answer suggesting you ask for referrals is a good one. Candidly, I'd put more emphasis on referrals and results they've gotten for clients than personal SEO. Find someone who has created the value you want for others. Too many "games" go on for positioning of industry terms, which just creates another opportunity to wind up buying snake oil.
Another option is to attend search engine marketing events like SES or SMX. The type of person or company who invests in their professional education at events like these is a good start at finding some reputable help.
Happy to chat through what channels are likely to generate the best ROI for your business - feel free to give me a call. Good luck!
Unless Google and Bing stop changing their algorithms there is no one that can call themselves an expert. The best way to find someone who can do for you what you are looking for is to
ask your network for referrals , or if you don't have a network to ask the consultant for references. Everyone has an area they specialize in i.e. E-commerce, Blogs, Hyper-local ; so be sure to get someone with experience who uses whitehat techniques to improve your SERPs.
Given Google's current direction, I'd worry less about SEO and focus more on creating fresh relevant content people will want to share online.
Sort of following up on some other answers here, I would stay clear of looking for a consultant via Google searches. Stick to referrals and social media. In finding a good SEO it's all about trust. Since so many can not be trusted, it's important to vet them in someway.
As far as referrals go here are some tips;
- Look up the reputable conferences such as Mozcon, SMX, SearchLove etc - look at the speaker list and start by contacting them. They will likely be too busy but they should be able to refer you to someone.
- If you know any designers, developers, CRO, UX people - ask them.
For social media, what I mean by this is - start looking around Twitter and Google Plus for SEO's that seem to have a good following, share valuable content, have a good attitude, and are generally helpful. Follow some people for 2-3 months to really get to "know" them.
I would also look at the helpfulness and quality of content published by SEO's. Their blog posts and articles will generally be indicative of the quality of work they do.
Ask for anyone you may want to contact to provide referrals of clients that you can pick up the phone and talk to.
To be honest, as an SEO consultant myself, I can attest that the WORST leads come from ... ironically ... SEO (someone Googling "seo services" and then emailing me). I wish I didn't rank #1 for "SEO Massachusetts" because this has only been a source of poor leads.
Finally, as others have mentioned Clarity does have some of the best SEO's on their netork. For example I can attest to knowing John Doherty and Nick Eubanks personally and their work and professionalism is top notch. Just look up any of their blog posts and you will see what I mean :)