Hi there, I have a 2004 XRT6 sedan bought 2nd hand. The rear shocks were noted as leaking in a previous service - probably a few services ago now & I am planning on replacing them myself.

How can I determine whether the previous owner has lowered it from standard?

If the springs are black and they have a yellow tag with a number, you can be 99.9% sure they are genuine. If they are Blue or Yellow or Red, then, they won't be genuine and you will need to check the height of the car to see:

Standard with 17" wheels
Front - 630mm from the bottom of the stock rim to the guard
Rear - 635mm from the bottom of the stock rim to the guard

thanks for that, turns out the springs are yellow. Even better was when i removed one of the wheels and the 'King Springs' logo with part number was visible.

Looks like fronts are still original so that'll be a future upgrade.