How to check percentage yields in a given reaction?
They are determined experimentally. There isn't any possible way you can determine percentage yield by looking at a reaction (unless there is only 1 possible product).

What the text is trying to explain to you is that when comparing analogous reactions of $\ce{Cl2}$ and $\ce{Br2}$, there will be higher percentage yields of the more highly substituted (and stable) product in the $\ce{Br2}$ reactions. For example, in the reaction between propane and $\ce{Cl2}$, the yields of 1-chloropropane and 2-chloropropane are 45% and 55% respectively. In a reaction between propane and $\ce{Br2}$, the yields of 1-bromopropane and 2-bromopropane are 3% and 97% respectively.

The exact reason for these selectivities has to do with the differences in energies in the transition states for the formation of the free radicals involved in the mechanism for these reactions. If you're interested in learning more you can see for yourself here.