Nature of the Bonds

Let's take a look at the nature of each bond in DMSO.

To find the nature of each bond, take the difference of the electronegativity values for each element in the bond.

Electronegativity Values:

H: 2.2
C: 2.5
S: 2.6
O: 3.5

A difference of 0 indicates a absolutely covalent nonpolar bond.
A difference greater than 0 but less than 1.7 indicates a covalent polar bond.
A difference greater than 1.7 and less than 3.5 indicates an ionic bond.
A difference of 3.5 indicates an absolutely ionic bond.
A difference of 1.7 indicates a covalent and an ionic bond.

O=S: 3.5 - 2.6 = 0.9

The double bond between oxygen and sulfur is a moderately covalent polar bond.

S-C: 2.6 - 2.5 = 0.1

The single bond between sulfur and carbon is an extremely covalent polar bond.

C-H: 2.5 - 2.2 = 0.3

The single bond between carbon and hydrogen as a very covalent polar bond.

--Note that all the bonds are covalent polar bonds.