@kaniini published at VPSboard nice little tool to probe an OpenVZ/LXC environment and see if it is slabbed and I thought to share it also here.
So far I found two popular providers who run OpenVZ stuff inside KVM and Xen. One of them stated publicly several times that they don't do this :)
https://github.com/kaniini/slabbed-or-not
Compile and run with:
wget https://github.com/kaniini/slabbed-or-not/archive/0.1.tar.gz && tar zfx 0.1.tar.gz && cd slabbed-or-not-0.1 && make && ./slabbed-or-not