ffmpeg -i my-audio-file.flac -af ebur128 -f null -
...should output something like:
Summary:
Integrated loudness:
I: -16.2 LUFS
Threshold: -26.7 LUFS
Loudness range:
LRA: 3.6 LU
Threshold: -36.8 LUFS
LRA low: -18.2 LUFS
LRA high: -14.7 LUFS
Where LRA high is the peak loudness of the file.