Google's Gary Illyes shared on Twitter that although Google currently uses mostly desktop factors for mobile rankings, it doesn't mean Google is dumb and only trusts what the desktop web site says, despite whatever might be going on with your mobile site.
Gary Illyes said that despite all of that, it "does not mean we will not check for huge discrepancies between the two."
That means, Google does check and if there is enough of a difference, they probably flag it for a more careful review.
How does Google determine when there are "huge discrepancies" between mobile and desktop? I doubt he will share that, but I suspect that will turn into something more automated over time. For now, I suspect they flag these cases and send a manual action reviewer (web spam fighter) to take a closer look.
@jbobbink We're currently using the desktop version for mobile (which does not mean we will not check for huge discrepancies between the 2)
— Gary Illyes (@methode) July 28, 2015Forum discussion at Twitter.
Update: How different is vastly different? Here is a tweet from Gary:
@brianrutledge more like "adopt this puppy" on the desktop, and "buy Viagra online in a casino without prescription" on mobile @rustybrick
— Gary Illyes (@methode) July 29, 2015