[App_rpt-users] Way to detect activity?

Is there a simple way for an external script to detect when a node is active or not? I'd like to create a script that plays back a message, but only when the node has been inactivate for some period of time and then occasionally once it is active. Similar to how WAN RS does theirs, but I'm pretty sure they use a cron job to do that. I'd like to just detect activity and so it based up on that. Thanks in advance, and any pointers appreciated. Alan