bsdlabel -e /dev/ad4s1

# /dev/ad4s1:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a: 40960000        0    4.2BSD     4096 16384    64
  b: 1912565105 40960000      swap
  c: 1953525105        0    unused        0     0         # "raw" part, don't edit