The ability of an atom to draw electron density towards itself is called its electronegativity . You need to memorize the relative electronegativities of these elements so that you can assess the polarity of covalent bonds. Whenever a bond connects two atoms of different electronegativity, the bonding electrons will be polarized towards the more electronegative atom and the bond will be a polar covalent bond. The polarity of the bond is determined by the electronegativity difference between the two atoms. If the difference is really large, as in LiF, then the bond will be ionic.
Smaller differences give polar covalent bonds.
Bonds between C and N, O, F are all polar covalent. C is slightly more electronegative than H, but the difference is usually so small that CH bonds can be considered nonpolar. Many factors affect an atom's electronegativity, including its formal charge. This effect should be regarded as a small, but important, modification of an atom's intrinsic electronegativity. For example, OH bonds should always be regarded as polar covalent bonds.