Of millions (I think)and even more SEO companies worldwide, there are only few which are worth trusting. Since in this industry where majority is a scam, choosing the right SEO company can be very tiring and difficult.
Here are some few points or characteristics of a scam SEO company to have you warned and avoid them.
*They offer impossible or unattainable guarantees. A guarantee such as Top 1 ranking in all major search engines even if your site is cluttered (with nonfunctional or broken links throughout the page) and not attractive at all, is definitely a sign of scam. Higher ranking in the search engine listing can only be possible if the web design itself possesses the basic principles of web designing: visually pleasing and functional. Top 1 ranking for particular keyword phrase is possible but for several months to achieve through white hat strategies. Those that offer just a week or a month is a scam.
*They only change Meta tags or do onsite optimization while not doing offsite optimization. Stuffing the Meta tags with your target keyword phrases is a spamming technique. Before this happens, quit from the contract with this kind of SEO company before anything gets damaged too late to repair.
*They create doorway pages that are hosted by another server. This is a tactic to have your site be able to generate more web traffic. You are getting a lot of traffic but no returns on investment at all. As a result, you keep on paying the SEO company for the voluminous web traffic they generate for your site while not getting any sales.