Hi there, I am not an expert on the subject , but let me give my two cents. Skill assessing authorities usually does not mention the years of experience on their letters , except for ACS , I had my skills assessed by Engineers Australia , and they didn't mention the number of years, they do it seperatley and there is a fee for that .
DIAC is the final deciding authority for the number of relevant years you have worked , regardless of the outcome by assessing bodies.
For proving your work experience , you will need to show salary slips, bank statements, work experience letter from your employer, if you are self employed you can also show the payments being made to you as a proof of your work experience.
Sorry for typos , and please correct me if any one have a better idea about claiming points for work experience issues.