SEO with no Smoke or Mirrors

How can a company know if the Search Engine Optimisation company that is pitching to assist is really any good and is equipped to help them in their drive for higher traffic? Also, how can a client make sure that the works carried out in their name are OK and are going to help and not hinder any progress? All of this is very hard for the Non SEO professional as the area is a very complex (and constantly changing). What companies need therefore is someone that they can trust. This site seems to give a lot of answers