We have seen that sometimes forcing your opponent to substitute his king for another piece will create a working skewer. The same goes, of course, for forced substitutions of other pieces. Take the position to the left. White has no loose pieces and Black has no checks to give. What remains as a source of tactical ideas? Captures. In addition to considering any checks you can give, look piece by piece for any exchanges you can force. Here the only capture available to Black is NxB. Unless he is content to lose a piece, White has to respond with QxN. The question is how the board will look after the exchange, and whether anything not possible now will become possible then. In this case we see a now-familiar pattern: White’s queen would be lined up with the rook on a1 on the long dark diagonal. Black still has his dark-squared bishop, and he can get it onto the vulnerable line with Bg7; now the queen must evacuate and allow BxR. Black doesn’t care that the rook on a1 has protection, as he still wins the exchange.
You might also have seen this just by observing the alignment of White’s bishop and rook on that same diagonal and looking for a way to substitute a better piece for the bishop. This isn’t an easy train of thought, because by itself a bishop in front of a rook often will not signify anything. It nevertheless is a good habit to notice every time two pieces are lined up like this, so observe the pattern closely.