howto check if a conversation window is active (on a regular base)

[suckme2000 at gmx.de]

Hi there!
I am writing on a plug-in and i need to check if a conversation is still 
not active or if it has become active again.
For example,
A writes to B , but B is doing something else
While B is doing something else, not paying any attention to the 
conversation.
The plugin should continuesly do something, for example send an audio 
signal *beep* until B activates the conversation.

Is there any signal that i can check for, with 
purple_signal_connect(...) or any other way to that ?
It would be nice if anybody could answer my question.

Fred