I'm not even sure what made me look up this rule, but it's actually pretty simple:
A contraction for who is or who has
For example: Who's coming to dinner?
For example: Whose grandmother is that?
The best way to fix this is to replace the word with who is or who has.
Who is coming to dinner? Yes.
Who is grandmother is that? Um, no.
I saw one error, which made me notice the discrepancy in the first place, then I thought there could be others, so I did a simple search for the word "who" in Scrivener (this can also be done in MS Word). It pulled up a bunch of other words (whole, whom, etc.), but I was able to quickly scroll through and determine which of these were misused.
And yes, there were others. *sigh*
We are all fallible. :)
P. S. I don't know how many stories I've sent into this world with such a simple mistake. If you locate one, just don't tell me. I'd rather not know. :D
P. P. S. Boring post, I know. Sorry!