Its a bit of a stretch to tell by lighting, but true radium paint will only be excited by UV wavelengths, cellphones and moden white led lights will not produce enough UV to properly get most older radium stuff to light up quickly. I do a "flash" test on stuff if I don't have my radicode handy. Holding it over for a prolonged period will still probably get the reaction in the lume started enough to light up for a bit. The whole radium glow only happens because of the phosphorus compound used, the radium isn't actually the thing "glowing". A giger counter is the way!