I do not think you need to provide a proof unless it's regulated. It's kind of imperfection of EE System. IMHO it should ask if you were required to obtain qualification for your NOC and then ask this question. if NOT system should account all your points towards experience..It’s not regulated also in terms of government policy...
but most importantly, is that question (when did you get qualified for your occupation) important? And do I also need to provide proof for that?