Set realistic expectations
Be very suspicious of anyone who promises you top 10 ranking. Ask them how they can be so confident. Ask them how competitive they think your particular industry is. And most importantly, ask them what they are going to do to get you results – if all they talk about is top 10 ranking, first of all, it’s unlikely they will achieve it and second it’s just not enough to achieve results.  
Check their previous work and client satisfaction.
Ask them to give you a couple case study examples showing what they have done for other clients. What is good about the sites they produced  — ask them to explain the thought behind the design, navigation, content, etc. Are they truly ranking well for good keywords?
What kinds of suggestions are they talking about for your site? Does it sound reasonable? Do you understand their explanations?
Be sure that it looks like they are talking about both improving results AND improving your visitors’ experience on your site. A site that does well in the search engines and then disappoints its visitors is not worth any more to you.
Does the consultant tell you what they’re doing and why they’re doing it, or do they act as though it is secret or beyond your ability to understand?  
This can be very frustrating. You hire a consultant but don’t really know what he is doing and he avoids providing meaningful explanations.  You don’t need to know every detail of what the firm is doing but you should be receiving a satisfactory explanation about their methods, expectations and goals. 
Does the consultant present you with lots of sophisticated looking automatic tools?
There are good solutions out there and they play an important role in Internet marketing and ecommerce. But anyone who tries to automate everything — the development of content, navigation, link campaigns, search engine optimization, etc. is not going to do an effective job. Sites need to be built by people for people.
Just as brick and mortar marketing can not be automated, Internet marketing can’t either. There is a lot of creativity involved – and a good consultant will know how to properly integrate the art with the science of it.