You need to check the checkbox's state in the OnClientBeforeShow event of the ToolTip and set the desired text accordingly. For your convenience I have prepared and attached a simple page that does that.
Please note that using the OnClick event as a ShowEvent of the tooltip would cancel the checkbox's default behavior (becoming checked/unchecked on a click). You can test this in the demo page I have attached. I would recommend using the OnMouseOver event instead.
the Telerik team