Elements and Compounds

it has more electrons in the product than in the reactant

It is determined from known oxidation states of other elements.

the number of electrons the element needs to lose or gain to have a full valence shell

In an atom of an element has one or two valence electrons and is not a helium atom, the most likely oxidation state is positive by the number of valence electrons, while if the atom of an element has seven or six electrons, the most likely oxidation state is negative by the number of electrons required to total eight valence electrons. Helium and all the atoms with eight valence electrons have a most likely oxidation state of zero, corresponding to no reaction.