The fastest way of determining the UUID of a LUN on ESX is through /proc:

# grep "Id:" /proc/vmware/scsi/vmhba1/8:5
Id: 60 5 7 68 1 90 81 1a d8 0 0 0 0 0 0 34 32 31 34 35 20 20

This corresponds to a UUID of 600507680190811ad800000000000034

On ESX 3.5 go to /vmfs/devices/disks to see the mapping to a vmhba.