Question: How do you know which address is the change address?
Your client knows which part of the transaction is the change because you own the key for that address.
From the outside you have a 50/50 chance AT WORST assuming you're looking at a tx from an official bitcoin client. My guess is most of the time you have a much better chance at guessing which part of the transaction is the change than 50/50. For example, on a transaction where the payment is 1 BTC and the change is 2.0291 BTC, it's pretty obvious which part is the change. I haven't done too much research on how often change amts are obvious but I just decided to be conservative and assume they're always identifiable.
Still, that would be an interesting project to undertake.
Haven't checked my home computer's wallet yet.
