It would be “who’s” — “who is”.
Spelling in context is one, another is use of expressions or repetitive patterns.
For example if I were to say "Mike Stoops is a stupid football coach. Very stupid.", you, as an observant person would notice the repetition as a way of emphasizing a point.
Tomorrow, someone could ask you if you know anyone who makes a statement and then briefly repeats it, or emphasizes it, with a following declarative that says the same thing.
"Well, yeah, there's this bloviator on FR who's (whose?) been saying stuff about some football coach and as a matter of fact he sometimes struggles with spelling in context too."
Gee... we've narrowed it down some...