I'm sure if Walt was anyone other than his brother in law, Hank would be on to him, but his personal feelings are still clouding his judgment, I think. Plus, he bought the infidelity story.
The pieces are all there, Hank just needs a nudge. Walt's a chemistry teacher, really smart, is suddenly "leasing" new cars and expensive watches, and conveniently had a gambling problem that paid for his whole medical treatment.
He brought up Walt's second cell phone last week. All he needs to ask is "hey, wait, Walt wasn't the one who cheated, why DID he have the 2nd cell phone?" and/or someone commenting that the master chef needs to be really good at chemistry.
The fact that they brought the 2nd phone back via Hank's dialogue and made a big deal of Hank/Walt talking about his new bling tonight, I think indicates that we're coming awwfully close to Hank finally putting it together.
That's my bet for the half-season break cliffhanger. Hank's drop-the-coffee-cup moment