Front Discs - when to replace

[23skip]

Car is due in at dealer for service in a couple of weeks time (apx. 23000 miles done). I am aware it will need new front pads as the light has just started to come on occasionally and they are looking a bit thin. However can anyone advise how to judge if new front discs are required. Mine have a slight lip (apx 1mm) but other than that seem fine. Just want to be sure nothing is done by dealer if not necessary.