The real key to spotting a powerful mage is using a good foundation before dabbing on the black or white paint. It doesn't have to be something stark, pure white (or stark, pure black, depending on skin tone). (Stereotypes aside, most powerful mages are not that goth.) But something that highlights the contrast. Also, depending on the mage's personal standards of modesty (and another stereotype: most mages are not into all that "dancing skyclad" or Aleister Crowley stuff), you might get them to wear briefer clothing, maybe shorts and a tank top, so they have more skin to spot. Ask them about allergies and sensitivities before applying foundation and paint.
With these simple precautions, you will succeed in spotting your mage, which is often a fun and ebullient thing to do for special events!
Brevity Quest 2021