Finding an SEO consultant that understands your business online can be a challenge, however if you cannot find him or her in Google then perhaps this could be a useful warning sign
Simple, search Google. If the consultant is worth his salt he will appear in the top 10 results. If he doesn't will your site ever using him / her?
Another check is the SERPs for keywords like web marketing, internet marketing, SEO services and so on to see how your consultant fairs for the more competitive synonym keywords (SEO Consultant is fairly easy). If they are not on the first three pages then are they really any good?