This one is a bit visually deceptive. Where does White have the makings of a skewer? Not against Black’s king, clearly; nor is Black’s queen on any usable lines. Look for loose pieces. Black has one: his knight on a4; and in general two knights together on the same line, like two bishops or two rooks together, often are vulnerable and should cause you to think about a pin or skewer. Can White get a piece onto that line? At first it might not seem so; he has no way to get a rook in front of those knights. But he does have a way to get his queen behind them: Qa6. That's a potential skewer, though it's foiled by the Black rook on a8. White can't take out the rook, but there are other ways to get rid of a defender. One is to take something else the defender protects, thus prying it away from the primary target. So ask what else the a8 rook defends, and you are drawn to the rook on b8—which you can take, with RxR+. White plays that, Black replies RxR, and now the board is safe for White to play Qa6, skewering the two knights that both now are loose. He will win one of them on the next move.
A first thing to take away from this case is the queen’s ability to jump behind two enemy pieces to skewer them. Second, the position illustrates a general skill at the chessboard: rather than concluding that there isn’t a skewer, concluding that there almost is; seeing a near-possibility, so that you can start using your tools to make it happen. When you start to see a tactical idea, try to avoid saying “no, that doesn’t work.” Say, “that would work if only…” and then see if there is a way to take care of the “if only”.