How would you determine if the evidence you gathered is relevant and reliable?

Please write one page and half about the fallowing statement:
You have been assigned to investigate possible fraud relating to a discrepancy in inventory in which management has informed you they suspect the warehouse assistant manager is involved because the balance in the inventory account has declined by 30% over the past year and the assistant manager has been acting “suspiciously”.  What type of evidence would you gather both from within the company and outside of the company to support management’s theory? How would you determine if the evidence you gathered is relevant and reliable?  How would you obtain the evidence based on applicable laws and professional standards?  Please educative sources