Hi Denes,
Well, currently there is no easy way. vCenter events are not posted consistently in VCD 5.1 and even when they are, there might still be final reboot. The best you can do is described at http://blogs.vmware.com/vsphere/2012/06/using-a-guest-customization-script-to-tell-when-vappvm-is-ready-in-vcloud-director.html.
"needCustomization" is internal flag which indicates that something has changed and customization needs to trigger. In VCD 5.1 it's not the only one - there is also one such flag per NIC. So, it's all of them combined + "customization enabled" + Tools version >= 7299 + some other factors for particular corner-cases which result into decision whether customization needs to trigger. I strongly not recommend relying on those, since implementation may change at any point, even between "update" releases.
/Andrii