The btrfs format is now set enough that we can recognize it with file magic:

# BTRFS
0x10040         string          _BHRfS_M        BTRFS Filesystem
>0x1012b        string          >\0             (label "%s",
>0x10090        lelong          x               sectorsize %d,
>0x10094        lelong          x               nodesize %d,
>0x10098        lelong          x               leafsize %d)

This can go after the GFS/GFS2 magic

# mkfs.btrfs -l 16384 -L mylabel -n 8192 -s 4096 /tmp/fsfile
fs created label mylabel on /tmp/fsfile
        nodesize 8192 leafsize 16384 sectorsize 4096 size 1.28GB
Btrfs v0.16-37-gb8271dc

# file -m ./magic /tmp/fsfile
/tmp/fsfile: BTRFS Filesystem (label "mylabel", sectorsize 4096, nodesize 8192, leafsize 16384)

Thanks,
-Eric