I recently purchased some second hand adjusable rear drop links from Mattybr5 on here, I know they are for dialing out preload, but how would you know if you have preload?

The whiteline instructions just show how to install them.

Could anybody advise me please?

Thanks

 

With full weight on the car suspension - get a level on the arms of the rear bar, to ensure they're horizontal.
Then measure the distance between the centre of the required hole on the bar, to the centre of the hole on the mount - that's the measurement to adjust the link to.