Whenever I've asked that question with regard to property that I've been inspecting in Cairns the agent has looked in the meter box and been able to tell from the date that appears there. I presume that when power is first connected to a property there must be a requirement that the date is shown with all the other specs. It might be worth a look anyway.
yes, that's one way, or if it's a unit the date of registration of the strata plan or even have a look at the external garden tap! these are usually dated and are rarely replaced for some reason.