There are at least two ways to do this:

o Use "lvlnboot -v" (must be root); this command also reports additional
  information on volume groups and logical volumes:

  # lvlnboot -v `vgdisplay | grep "VG Name" | awk '{print $3}'`
           ...
           /dev/dsk/c0t5d0 (8/4.5.0) -- Boot Disk
           ...

o Use setboot. This command also reports the alternate bootpath, as well
  as whether Autoboot and Autosearch are enabled.  However, it does not
  print the disk device names as lvlnboot does.

  $ setboot
  Primary bootpath : 8/4.5.0
  Alternate bootpath : 8/4.10.0

  Autoboot is ON (enabled)
  Autosearch is OFF (disabled)

To determine which disk the currently running kernel was booted from, use
adb as follows (must be root):

  # KERNEL=/stand/vmunix  # set this var to the currently running kernel
  # echo 'boot_string/S' | adb $KERNEL /dev/mem | grep /
    boot_string:    disc(8/4.5.0;0)/stand/vmunix

On 11.x, the path of the currently running kernel can be obtained using
the kmpath command (see subject 8.2.4).