Question

how do you know if something is a metal or nonmetal

Answer 1

From looking at a periodic table. Most of them distinguish metals, metalloids, and nonmetals by different colors.

Answer 2

Like this one

Answer 3

just heat them at one end and see the effect on the other end

Answer 4

Metals conduct electricity. If you have a multimeter, you can measure resistance, which is very low.
Carbon is a non-metal and it is also a conductor, mut not as good as metals.
You can try it by using pen "lead"