Take a look at using IP SLA and EEM (Embedded Event Manager).
You can use IP SLA to track the connectivty from the backup router, generate a log message when on your device and send to sa syslog.
Or you can use EEM to check the logs for the up/down log message of your ATM/Dialer interface, then perfom some action (like shut down the link to your other router if that's what you want), or even send an email to you.
IP SLA
http://www.cisco.com/en/US/docs/ios/ipsla/configuration/guide/sla_redirect.html
EEM
http://www.cisco.com/en/US/products/ps6815/products_ios_protocol_group_home.html