From Portal:
1. Login to the AWS Management Console.
2. Select direct connect service and go to virtual interfaces tab
3. Verify if any public virtual interface is associated with any instance.
4. Make sure to fix the configuration to avoid public internet routing through your direct connect interfaces

References:
1. https://docs.aws.amazon.com/directconnect/latest/UserGuide/WorkingWithVirtualInterfaces.html
2. For creating private virtual interface: https://docs.aws.amazon.com/directconnect/latest/UserGuide/create-vif.html