Guessing in practice you would melt some and see if it's shiny (leaded) or chalky (lead free.) Surely not foolproof for all alloys though. Otherwise you could

Guessing in practice you would melt some and see if it's shiny (leaded) or chalky (lead free.) Surely not foolproof for all alloys though. Otherwise you could see how hot your iron needs to be to make it melt.