Interesting because all of the brokers provide very little information to go off of.
Unless you are making offers and getting names off the contract to then look up their deed, if you see they bought early you continue with the deal, if not you cancel the transaction. The downside is paying to look up the deeds, the time commitment, and jerking the brokers around if you do it to many times.
The only other thing I could think of is
www.fidelityrealestate.com lists the unit number, and you are targeting some of the earliest units that were sold, then possibly going through the above step anyway.