Make sure Toolbars->Levels (and Mixer) is checked or click the button. Now the mixer you have selected in the sound card settings, if any, will be displayed. Shown below is the WinMLS mixer.
Set the volume sliders to a low level as shown above and connect the device you want to test. Note that you can use the keyboard arrow keys to do small volume changes, but you must first click at the volume slider to set it active.