The indication that an internal auditor was assigned to an assurance engagement is most strongly given by the requirement that the auditor must not assume management responsibilities while performing the engagement.
This aligns with the principles of objectivity and independence, which are critical in assurance engagements to provide unbiased and reliable conclusions. Taking on management responsibilities could compromise the auditor's objectivity by involving them in the operations they are auditing.References: IIA's International Standards for the Professional Practice of Internal Auditing.