This does the trick:

ffprobe YourVideoFile.mkv  2>&1 | grep "Subtitle"

which generates a list like so:

  Stream #0:2(eng): Subtitle: subrip (srt)
  Stream #0:3(eng): Subtitle: subrip (srt) (original) (hearing impaired)

Or more advanced:

ffprobe -loglevel error -select_streams s -show_entries stream=index:stream_tags=language -of csv=p=0 YourVideoFile.mkv

Which generates a more concise list:

2,eng
3,eng

Or if you only want to see the English (eng) subtitles:

ffprobe -loglevel error -select_streams s -show_entries stream=index:stream_tags=eng -of csv=p=0 YourVideoFile.mkv

Which generates:

2
3