A -6 dbFS sweeping sine tone, from 10 Hz (supposedly inaudible) to 200 Hz (supposedly played back by all sound systems, including those smallish laptop speakers). On the top of the test tone, a voiceover tells you which frequency is currently playing.

Play back the file until you start hearing the underlying sweeping tone as it rises. The voiceover tells you the frequency you have reached. This frequency more or less represents the lowest limit of your audio system.