Having witnessed Iranian military readiness first hand, the indicator that Iran is preparing for the scale of operation required to attack targets in Saudi Arabia would be very obvious to any outside observer. Any outside observer would not require the breadth of technical ability available to western intelligence agencies (such as satellite intelligence). Unfortunately for Iran, this is one of the pitfalls of basing your military aviation assets alongside your civilian aviation infrastructure. You are able to see the tempo of training that the Iranians maintain, and the state of their jets, quite freely at most regional airports.