It depends some on how you have this
configured. If you are using the backup
command, you can first remove that command
and then generate interesting traffic to
cause dialing, to ensure that it dials out
and connects. To check that the backup
actually works, you would have to bring
down the primary link.
If you are using dialer watch, you would
have to manually cause the watched route to
disappear to verify that the backup
works.
If you are using floating static routes, you
could simply generated interesting traffic
to verify that the DDR portion works OK.
