How can you find out if an Internet account is being used to download movies illegally?

The MPAA and RIAA work closely with third party monitoring companies who keep track of illegal downloads on sites like Pirate Bay. Cable companies also monitor bandwidth activity of their customers and can send up to six warnings to those suspected of illegal downloads. If the activity continues, the provider can slow down the Internet bandwidth or require that the customer is educated about downloads before allowing continued access to the internet.
Customers who wish to monitor the bandwidth being used by an account can do so by logging into a service provider and using online tools that show much data is being used, according to HowToGeek.com.
Other options for keeping track of bandwidth is to download a third-party program that tracks activity over a network through its router. Firmware programs like DD-WRT allows a user to track all of the activity on a network instead of just one computer.