Connect a pair of head phones to the output of your interface check for audio. Sound Blaster can also make a good recordable oscilloscope. Chances are if you are reading this you at least have a computer with a sound card with wave editing software that came with the drivers. Hook it through the line input and record the data like you would audio and zoom in on it. It should look like the picture.
Line testers are also helpful.
Remember oscilloscope tells all.