MOTM

Yup, an excellent technique. You can also synthesize frequency sweeps in the
computer and use those as the test signal. There is a freeware program
called MakeATestTone which generates AIFF files of frequency sweeps of
whatever duration, range, and amplitude you want. Play the test tone out
through one channel of your audio interface and record the output of the
device through an other channel.

I suppose the first thing one should do when employing this technique is to
loop the test tone output straight back into the recording input, so as to
measure the frequency response of your audio interface.

--Adam