The businesses that have been online for some years will have the advantage of knowing more than newbies and of course that is true of any field.
Experience counts for a lot, and you don't want to be involved with someone who can't make ideas work for clients.
Ask for a list of sites that the company has worked for; they may not provide you with this information as they may be the father confessor types who don't tell who their clients are or have been.
However the reputable ones won't mind sharing the top names they have worked with.
You can then do a little digging to find out if what they claim is true.
Become a member of sites that have information you need about SEO.
Most allow you to subscribe to basic level membership for free and then you can decide for yourself if you want to pay to join the forums which require paid membership.
Just type the information you require in the search engine box to get a list of top sites.
When you join a forum, start asking about SEO companies and which (if any) other people recommend- although the recommendations may come from SEO company people of course.
SEO is clearly the way to go to attract more visitors to your site, and the best way of rising in the SERPS is for your site to have interesting original content to keep people who are, after all, potential customers and clients, on it.
The more pages it has, and the more original content, written in clear concise English, the better.
For non-native speakers of English this latter fact is daunting, but if you want your business to expand and to increase traffic flow to your site it is necessary to have content written in good English; otherwise visitors will be put off and that is sad, but true.
Good SEO firms will be able to help you produce good content or be able to find you a good content writer.
They will also be able to help with uploading and making videos to post on your site, and these are being more widely used by online marketers to promote their products.
You need a good SEO company to ensure your success in the field of Internet marketing is you want to have a visible profile in the web.