Why are the dispersion forces in CS2 stronger than the dipole-dipole forces in COS? London dispersion forces supposedly have the least strength out of all the intermolecular forces. But $\ce{CS2}$, which has only dispersion forces, has a higher boiling point (and thus stronger intermolecular forces) than $\ce{COS}$, which has dipole-dipole attraction in addition to dispersion forces. Why is this?

I suppose that it has something to do with $\ce{CS2}$ having a thicker/more inducible electron shell, but then a new question arises: how would you know if the dispersion forces in one molecule are stronger that the dipole-dipole forces in another? 

(Theoretically, without using boiling points or other experimental data. Also, this is based on question 4a from the 2018 AP chemistry free response.)
Although individual dispersion forces are weak, they are cumulative, and increase with molar mass.  As a general rule, boiling point increases with molar mass. 

Polar molecules will have higher boiling points when compared to molecules with similar molar masses.  For example, ethanol($\ce{CH3CH2OH}$) has a higher boiling point than dimethyl ether ($\ce{CH3OCH3}$).

$\ce{CS2}$ is ~16 g/mol heavier than COS.