King Air C90 Propeller Balence
Let me say from straightaway that I'm not a fixed-wing person but I believe my experience with helicopter rotor balancing gives me some limited authority to suggest an answer to your query.
Both prudence and 'best practices' dictate that you should perform dynamic propeller balancing upon installation of a new prop and reinstallation of the existing unit. Don't forget to index the prop to the flange and clearly highlight the attached balance weights so that other maintenance personnel don't move them. Subsequent to this, further dynamic balancing should be performed whenever you have a suspicion that it is required.
If the aircraft is flown by a number of different pilots, sooner or later one of them will comment on the propeller vibration level but if you're the only pilot then you may not notice the gradual rise in the vibration level and this is when it's best to establish an interval to accomplish the task. I would suggest you start at 200 hours and see what you get. If the IPS is essentially unchanged then extend the interval to 300 hours and so on.
One point to note, when you're performing the dynamic balance DO NOT stop when you reach the 'acceptable' IPS - keep going and get it as low as you possibly can - the airframe will thank you for it - a truly smooth rotor/propeller greatly enhances the reliability of structure and components.
Finally, your question "How do I determine the IPS with out carrying out a dynamic balance?" I believe the only way is to put the equipment on and do a run.