Question

How can you tell if an element is a metal or nonmetal?

Answer 1

If you look on the periodic table (especially one online) you can see that it is colour coded...so this might help...

If you look you can see a "staircase"..which consists of B, Si, Ge, As, Sb, Te, Po...If it is on the lefthand side of the staircase it is a metal (ie: Ni) or if it is on the righthand side it is a non metal. There is lots of images on google that can help point it out.
 
If you are talking in terms of properties you should look for (Metal) materials that are: 
Lustrous
Hard.
High density  (heavy for their size)
High strength (resist being stretched)
High melting and boiling points.
Good conductors of heat and electricity.

Non metals are/have:
High ionization 
High electronegativities
Not malleable or ductile.
Little or no metallic luster.
Gain electrons easily.
Dull, not metallic-shiny
poor conductors of heat
and poor conductors of electricity 

Hope this helps!

Answer 2

Do a electricity test if you have the proper equipment 
you basically see if electricity runs through or conducts in the element<span />