Testing DSL modem without DSL circuit

I have come into possession of a Cisco 678 DSL modem that I'd like to sell. However, I am not certain that it works so I'd like to test it before selling it. My problem is that I don't have a DSL circuit to plug the modem into -- I use a cable modem. Is there any way, besides the power-on self-test that the modem does, to check it without plugging it into a live DSL circuit?

Thanks!