Polarity observed when a stream of liquid subjected to charged rods.  Nonpolar CCl₄ is not deflected; moderately ploar acetone deflects slightly; highly polar water deflects strongly.

The measure of molecular polarity is a quantity called the dipole moment (u). Dipole moment is defined as magnitude of charge (Q) times distance (r) between the charges.

u = (Q)(r)       Q charge in coulombs (C)      r distance in meters (m)

Consider a proton & electron 100 pm (10^-12 m) apart: 
Each particle possess a charge of 1.60x10^-19 C.  When proton & electron closetogether, the dipole moment (degree of polarity) decreases. However, as proton & electron get farther apart, the dipole moment increases. In this case, the dipole moment calculated as:
u = (Q)(r) = (1.60x10^-19 C)(1.00x10^-10 m) = 1.60x10^-29 C^.m       [1 debye (D) = 3.336x10^-30 C^.m]

The debye characterizes size of dipole moment. When a proton & electron 100 pm apart, the dipole moment is 4.80 D:
(1.60x10^-29 C^.m)(1 D/3.336x10^-30 C^.m) = 4.80 D

4.80 D is a key reference value! It represents a pure charge of +1 & -1 100 pm apart.
The bond is 100% ionic!
 

Note:  The debye named after Peter Debye (1884-1966)
who was awarded 1936 Nobel Prize in chemistry
for studies of dipole moments.
 
 
 
 

When proton & electron separated by 120 pm, u = (120/100)(4.80D) = 5.76 D  (100% ionic)
When proton & electron separated by 150 pm, u = (150/100)(4.80D) = 7.20 D  (100% ionic)
When proton & electron separated by 200 pm, u = (200/100)(4.80D) = 9.60 D   (100% ionic)

It is relatively easy to measure dipole moments. Place substance between charged plates--polar molecules increase the charge stored on plates and the dipole moment can be obtained (has to do with capacitance).
 
 
 

CH₃Cl:
C-Cl, the key polar bond, is 178 pm. Measurement reveals 1.87 D.
From this data, we can compute % ionic character.
If this bond was 100% ionic (based on proton & electron),
u = 178/100)(4.80 D) = 8.54 D
Since measurement 1.87 D,
% ionic = (1.7/8.54)x100 = 22%

HCl:
u = 1.03 D (measured)     H-Cl bond length 127 pm
If 100% ionic,
u = (127/100)(4.80 D) = 6.09 D
% ionic = (1.03/6.09)x100 = 17%