As with any system is is important to test that the backup server functions as expected. Your testing regime should include a full production test including having any automated aspects run their due course. Although this will result in some disruption of service to users it is better for a brief outage to occur under controlled circumstances than for some unexpected behavior to surface in a crisis situation.
It is also a good plan to have a regular testing procedure in place. The nature of the backup server is that it hardly ever gets used and is likely to be used for other purposes from time to time. As such it is very easy for one configuration or another to get altered and go unnoticed. By conducting regular, possibly automated, tests you can ensure that the backup server is always in good shape.