Its not a precise science as it depends on the sort of driving the car has done and the drivers style, but after 23,000 discs and pads should still be good, as Rich says check for lipping on the disk, run your finger from the clear metal area outwards off the edge if its heavily pronounced then they might be warn but doubtful with the mileage. Pads you should be able to see the thickness the car also has wear indicators so unless your seeing odd wear patterns you should be fine.
Only other thing to consider if the brakes feel crap is when was the last brake fluid change carried out should be every two years as water seeps into the system, also if the cars an Auto you can get funny things happening to the pads as they drag on the disk more such as glazed/hardened layer so breaking force will be reduced.
