There definitely is a way to know if that primary offer has fallen through. When you submit your back-up offer, have your agent submit the CAR form P.A.A. This is an addendum to the purchase agreement. There is a space on the second page that contractually indicates that you are back up offer # _______(fill in the blank). We use this when we make offers on properties and also when we accept offers on short sales as so many fall out. It helps everyone to know exactly where they are.
Good luck. I have purchased both of my homes by being the back-up offer! So many escrows fall out these days - it is a smart strategy.