Besides referring to the periodic table to know that sodium is a metal and sulfur is a non-metal as the elements in Group (I), (II) and (III) of the periodic table is metal and Group (IV),(V),(VI) and (VII) of the periodic table is non-metal. We could also refer to the number of electrons of the element.

Sodium (2.8.1) is a metal because it will lose 1 electron to complete the valence shell( to make it stable) and conduct electricity. Sulfur(2.8.6) is a non-metal because it will gain 2 electrons to complete the valence shell (to make it stable) and it does not conduct electricity.

Additional: Group (III) is also known as transition metal or coloured metals. Group (0) is known as the noble gas as they can stabilise themselves.