You are giving an unknown element that dull in color. all you have to identify it is an electrical source and a meter that detects electrical charges. you would know it is a nonmetal if you turn on the electrical source and

Metals conduct electricity while nonmetals do not. Therefore if when you turn on the electrical source and the devise indicates a current, you know the substance is a metal. If you turn on the electrical source and the devise indicates no current, the substance is a nonmetal. I hope this helps. Let me know if anything is unclear.