You can use http://www.day.com/maven/jdiff-jcr1-jcr2/jcr-2.0/javax/jcr/observation/Event.html that has
PROPERTY_CHANGED Event example http://wemcode.wemblog.com/create-jcr-event. But problem with this is it might get called for last modified date and replication property as well. Also it is very difficult to pin point exactly which property change.
Other option is you can use audit log, From UI you can get this information from http://HOST:PORT/system/console/events
Sling eventing can you give proper information which property has changed (although you cannot subscribe to just such changes).