The easiest thing to do is a loopback at the CSU/DSU, assuming you have one. If so, connect the router to the CSU/DSU using the proper cable. Configure the CSU/DSU as you would to connect to a leased line. Do a local loopback at the DSU. If you want to test the CSU/DSU as well, make a loopback plug and insert it in the network interface jack of the CSU (clear the DSU loopback). Either of these methods should bring the serial interface up.