How to know if an element is more metallic?

The metallic elements are towards the left of the periodic table and it depends on how many valence electrons are left over (metals usually have a low attraction with valence electrons)

Metals tend to have a low attraction with their valence electrons. As you move down the periodic table, there are more and more electron shells in the atom, and therefore the further down you move on the table, the lower attraction the nucleus has to the outer shell electrons. As you move to the left, the nucleus is less positively charged (fewer protons), and so the attraction between the nucleus and the outer shell electrons also decreases. So, in conclusion, as you move down and left, elements become more metallic.