can usually tell just looking at them, just get familiar with what a set of new pads look like with regards to thickness in comparison to a used set, next time you get new pads ask for the old ones back and see the diff for future reference.
also run your finger over the disks from inside to out and see if you feel a lot of ridges, if so the same ridges will be mirrored onto your pads, a little is ok.
if you are not sure what thickness a new set of pads are just look at a new car at a dealership, any car and you will see the diff.
my wife drives her golf 200 miles a week with 50/50 roads and i usually have to do hers every 2-3 years, 1.6 golf though so gti will be harder.
never change the disks without changing the pads though as any pad scores / scratches etc will be transferred to the surface of your new disks