*First determine the number of electrons in outer shell of every element in the radicals.
This can be determined by counting the columns on periodic table the element from a noble gas.For example cyanide radical (CN) has 4 electrons in carbon atom and 5 electron in nitrogen atom. Then combine the atom with covalent bond so they share as many electrons possible without exceeding 8 electrons. For cyanide both carbon atoms can share 3 electrons. When nitrogen adds these 3 electrons to its existing 5 electrons it becomes 8 electrons called octet and carbon remains with 7 electrons.
*Now determine how many electrons are remaining to make all the elements in the radical stable. These remaining number of atoms is it's valency