How do we detect if the root disk is rebuilt or not?

When a rebuild is attempted the root disk is erased. So you can probably try to update the file-system then execute the process of evacuate and verify that the update on the fs is still alive.

  nova boot --flavor 1 --image cirros i1
  guestfish -a /openstack/instances/775613dd-3aa1-48e1-b1cb-05530f06c99a
  > run
  > list-filesystems
  /dev/sda1: ext4
  > mount /dev/sda1 /
  > touch my-file
  > ls /
  my-file
  bin
  ...
  > quit

  nova host-evacuate --target_host host --on-shared-storage
  guestfish -a /openstack/instances/775613dd-3aa1-48e1-b1cb-05530f06c99a
  > run
  > mount /dev/sda1 /
  > ls /my-file

I tried the suggestion above, and the file that I created through guestfish (/test-file) was still there, indicating that the disk image was not rebuilt from scratch.

Created attachment 1014014 [details]
Evacuation after creating file through guestfish

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0843.html