As a result of many Google AdWords account hijackings and the false charges that are often associated with such an intrusion, netmeg at WebmasterWorld has suggested some procedures to keep yourself in the clear and away from hackers. The list includes changing your password to your Google AdWords account and changing any associated client account passwords. Following that, especially for AdWords account managers, contact your clients and make sure that they are aware of the dangers of Google AdWords when accounts are being hacked into.
The above steps can help you have a "clear mind" with regards to your account, but the next step is to review who already has access to the account and cleaning out users who don't need to be there (or who aren't active).
Now, review those charges you have. Netmeg says that you should run a report on the account and review all charges to ensure that you know exactly where they are coming from. Do this on a regular basis (weekly, monthly, or whatever).
Since security is important and you definitely don't want to rack up thousands of dollars of charges due to AdWords break-ins, it is important that you have some sort of security strategy in place.
Forum discussion continues at WebmasterWorld.