There are a variety of commercial services that, for a fee, will monitor your credit reports for activity and alert you to changes to your accounts. Prices and services vary widely. Many of the services only monitor one of the three major consumer reporting companies. If you’re considering signing up for a service, make sure you understand what you're getting before you buy. Also check out the company with your local Better Business Bureau, consumer protection agency and state Attorney General to see if they have any complaints on file.
There are also now several free credit review services that provide access to this information without additional charge; do your research and check your own information regularly.