It's not clear what you mean by "without logging in to any api". All of the OpenStack APIs require authentication, because OpenStack is a multi-tenant environment. With appropriate credentials, you can use the command line nova command to retrieve the tenant that spawned an image like this:

nova show <image id> | awk '$2 == "tenant_id" {print $4}'

Since the cli tools just use the published REST api, you can obviously get at the same thing by either using the provided novaclient python module, as S.Ali suggested, or by writing your own code that talks to the REST api directly.

You can get information about the user and tenant that spawned an instance -- without authenticating -- by using the libvirt virsh command. For example, I have three nova instances running, corresponding to these libvirt guests:

# virsh list
 Id    Name                           State
----------------------------------------------------
 106   instance-00000069              running
 107   instance-0000006a              running
 108   instance-0000006b              running

The domain XML for each guest includes a <nova:instance> block that contains the following section:

  <nova:owner>
    <nova:user uuid="a570fa8d18824c4d867c0b4673b30324">lars</nova:user>
    <nova:project uuid="d4c6b93d16d14c4d9bbbcd2af0417bcc">lars</nova:project>
  </nova:owner>

There you can see both the user UUID and the tenant (aka "project") UUID.