To identify the atoms in a compound, first you should know something about the periodic table. There is a zigzag line that separates the metal and non-metal atoms in the periodic table. That zigzag line indicates the metalloids (B, Si, Ge, As, Sb, Te, Po). Any element at the right of this line is a non-metal and any element at the left of this zigzag line is a metal. There is an exemption with Hydrogen; though it is on the left of the zigzag line, it is still a non-metal element.
So we can now distinguish which atom in Ammonium Vanadate (NH4VO3) is the metal.
N -> right of the zigzag; non-metal
H -> non-metal; exemption
V -> left of the zigzag
O -> right of the zigzag; non-metal
Now we already know that, V or Vanadium is the metal in ammonium vanadate.