Recording Indicator in the MIDI Device Configuration Window
An external MIDI input device has a red recording indicator in the MIDI Device Configuration Window, as shown here:
The MIDI Device Configuration Window displays a recording indicator for a given device if there is a check mark for "Is MIDI Input Device" in the Device Properties dialog box for that device. See Specifying Properties of a Device.
When you are actively playing on the MIDI recording device (typically a MIDI keyboard), you will see the recording indicator light up with a bright red color. This is a good way to confirm if Composer is detecting your performance, as described in What To Do If Recording Does Not Work. Note that you can also monitor device recording activity in the Quick MIDI Device Configuration Window.