There are plenty of internal and external measures you can take to protect your company from wrong-doing — but odds are good you’re ignoring the best tool for the job.

Your company’s losing 7% of its annual revenues to fraudulent activity. That’s according to the Association of Certified Fraud Examiners’ (ACFE) “2008 Report to the Nation on Occupational Fraud & Abuse.” Apply that percentage to the total U.S. gross domestic product for 2008, and a projected $994 billion will be lost to thieves.

It’s not due to highly-skilled criminals in ski masks, either. In fact, if you want to see the next potential thief at your company, all you’ve got to do is glance at the next-door office or cubicle: 29% of fraud is committed by employees in the accounting department, while 18% is committed by upper management or executives.

Don’t think the clerk with the “Hang In There!” kitty poster has it in her? That might not be the best assumption: Only 7% of fraud perpetrators have had prior convictions, and only 12% have been previously terminated by an employer for fraud-related conduct.

Here are the top six ways employees can rip off a company (and some help telling whether it’s happening at yours):

1. Creating false invoices — 23.9% — If you’ve received a $10,000 invoice for an in-ground pool and aren‘t opening a company gym, it’s time to get suspicious (or at least a bathing suit).
2. Non-cash misappropriations — 16.3% — Next time someone asks if you can help them shove a Xerox machine into the back of their Corolla, make sure it didn’t use to live in your office.
3. Check tampering — 14.7% — Remember that odd feeling you had when Mary Wierzbowskivitch from A/P brought you three checks written to Wiezbowskivitch, Inc. for approval in one week? You should have listened to it.
4. Expense reimbursements — 13.2% — You should probably require receipts for any “business meetings” that took place at a bar at 4 in the morning.
5. Petty cash misappropriations — 12.6% — Keeping the petty cash in a piggy bank might be “cute,” but it’s just begging for mischief. It’s time for a padlock.
6. Payroll fraud — 9.3% — Got someone who claims they’ve worked more than 24 hours in a day? You might want to have a simple math lesson before showing them the door.

What’s the best way to stop fraud from hitting your company’s books? Internal controls and audits can only go so far. According to the report, almost half (46%) of occupational fraud was detected by tips from employees, customers, vendors and other sources.

What do you think? Is installing an anonymous tip line enough to catch — or deter — fraudsters who want to hit your company? Share your thoughts with us in the comments section below.