There is no way to know, beyond what you do. Cancel Delay will cancel any current delay in the rule that was marked Cancelable (but not those that were not). One way to accomplish a single-instance-at-a-time rule is as follows:
- Make 'Private Boolean is true' the Predicate Conditions.
- Set Private Boolean to false as the first action.
- Set Private Boolean to true as the last action.
Once you set PB to false, the rule cannot be triggered again until PB becomes true again.