We have two Asterisk servers. One is a warm failover (ie, we have to make it work manually should the first one fail, but it gets regular configuration updates so it's always ready to be made live) and the other is the live server. They both have Digium Wildcard TE405P's in them, and the hardware and software is otherwise identical, but the last time we needed to use the failover server, things went poorly when it was plugged into the PRI line. Now, we're upgrading the failover box to Asterisk 1.6 and pressing it into service.
I really need a way of thoroughly testing (including testing under load) the TE405 hardware and configuration before we need it. I can't just set up and tear down a PRI line for testing for an hour or a day, and the expense of the line prevents us from having a complete backup. Likewise, just switching the cable from one server to the next is also highly disruptive.
In the past for similar testing situations, I've just made a T1 crossover cable. The TE405P is a quad-port card, so on the failover system, just connect two of the ports back-to-back with a T1 crossover, then write a quick script to have asterisk place a bunch of calls out one port, answering them on the other port.
DR testing should be a part of your regularly occurring system maintenence plans, or at the very least, you should have regularly occurring DR testing annually, semi-annually, etc. You can do basic testing using the method ErikA suggested but you'll never really know if it works unless you test it with the real PRI line, making real calls. My recommendation would be to lobby for a regularly occurring DR test, scheduled for a time that's least disruptive to the business.
DR components are like anything else (backups for instance), they're of no use unless they actually work and in order to confirm that they work you have to test them.