Part of my engagement is to collect information from my client but I am not sure how useful it is. How do I know that what my client provides can be trusted?
This involved some circular logic. On the one hand, you are being hired for your expertise, independence and objectivity. These traits are what you use to identify the valid and useful information. On the other hand, you are somewhat bound to your client's view of the world and the facts on the ground. The client's reality and opinions of the staff are not something about which you would necessarily have independent information. Who are you to question facts and judgments presented to you by a client as the basis to give advice? I feel your pain.
Although it might vary between clients, depending on the nature of information and type of engagement, your trust in client-supplied information would improve if you set up a validation process. Consider categorizing collected information, gathered from company records, staff interviews, analyses by other consultants, commercial competitive analyses, media reports, etc. according to your confidence in the source. Note where information is corroborated and where it conflicts with other information. To the extent that information from a particular source seems frequently be inconsistent with other sources or a particular type of information has no corroboration, flag that as uncertain. Note which items are facts, which are opinions, and which are opinions masquerading as facts. Tip:
To the extent that supplied information is the foundation of your findings or recommendations, at least confirm that the information being presented to you is internally validated. Because your analysis hinges on the accuracy of this information, present your concerns to an internal team, specifically raising your concerns about accuracy and relevance. Discuss the implications of using data or information that may not be accurate and suggest ways to validate and the risks of assuming it is accurate if it is not. This lets the client take advantage of your expertise, independence and objectivity but still make the call.