Write a structural formula for the compound being identified, and then separate the atoms of hydrogen. Then do a count of the carbon atoms that are attached to the hydrogen atoms, and if it has a common bond with another carbon then it can be termed as primary. If it shares a common bond with two it is referred to a secondary.
Primary hydrogens are attached to a primary carbon. Secondary hydrogens are attached to a secondary carbon.
Primary carbons are carbons that are bonded to one other carbon. Secondary carbons are carbons that are bonded to two other carbons. Tertiary carbons are carbons that are bonded to three other carbons.