So how does an organization, serious about realizing the benefits of the ITIL guidance, go about discerning whether their prospective consulting firm has the experience necessary to steer them along the best practice trail? It's plain and simple:
Ask for References.
More than likely, an individual (even certified Service Managers) who has spent their entire SM career at one company (non-consulting) will probably not have the insight and experience necessary to effectively architect successful Service Management solutions for other organizations. My personal experience over the years has shown me that just because someone can talk the service management talk doesn't mean that they can walk the walk.