Purple lines are usually found on the lower abdomen, inner and outer thighs, outer buttocks, axillae and popliteal fossa and are usually purplish in colour, while those caused by glucocorticoid excess are usually wider than 1cm.
Other causes are usually less than 1cm in width, so you can check yourself in the mirror to see if there are any purple lines in these areas and seek medical attention if there are any abnormalities.