Couple days ago I found out that memory usage grow rapidly on one server. After couple minutes of investigation, it was clear that memory balloon take place. Of course it will happen only if you have vmware tools installed. To find out what was happening, you can look at vmware performance counters to discover how many pages were reclaimed from particular VM. But if you want to try to get these numbers from VM, you can simply

cat /proc/vmmemctl

This will only happens when you choose vmware tools modules, not provided with linux kernel (option --clobber-kernel-modules in vmware-config-tools.pl). Standard kernel module  vmw_balloon doesn't provides this feature, more about naming convention

http://comments.gmane.org/gmane.linux.kernel/1037049

Memory ballooning  also indicates that free memory on host drops below 4 %, it's first line of ,,defence'' in overcommited environment. Moreover it's not always bad when VM starts to balloon'ing, because when you have overprovisioned VM with plenty of free memory, you can take it back without performance degradation.