That would require your WES protocol to be stranded, which I haven't heard from yet. So that's the first thing to check. If you have a standard exome sequencing prep, strandedness is not preserved.

Secondly, does it make sense to look for strandedness in transcribed/untranscribed strand? Don't you expect a mutation to be present in both strands of the chromosome? Unless you're looking at an unfixed mismatch, but if I'm not mistaken these get repaired quite quickly.