The "cheater" way for identification of valence electrons in an element is to identify the Group Number of that particular element in Periodic Table.
Example:
Let Suppose we want to Identify the number of Valence electrons in Sodium and Oxygen.
Sodium:
Sodium is Present in Group-IA (1), means it has one Valence electron.
Oxygen:
Oxygen is present in Group-VIA (6), means it has six Valence electrons.
Valence Electrons and Chemical Bonding:
Number of valence electrons show the number of chemical bonds either Ionic or Covalent.
Example:
Sodium having one valence electron looses that electron and attain +1 charge, so it can make single ionic bond with any element having -1 charge.
Carbon has four valence electrons. It can make four covalent bonds with any four elements capable of making one covalent bond. Or carbon can make two double bonds with any element capable of making two covalent bonds.