Lisa: I separate an operational audit of Marketing from, as noted above, a regulatory compliance audit. If that's what you are looking for - - - stick to the basics of auditing: reconciled accounts, approvals for projects, approvals for invoices, budget, comparisons of budget to actual, ads that were paid for ran, billing procedures, expense accounts, control over goods (in this case ad copy, the logo, artwork), retention policies. You will also want to review any contracts and check their use of prepaids.