The way to determine which case is which on each layer is to look for opposite colours (i.e. colours that go opposite each other on the cube: red-orange and blue-green). Imagine you can see two opposite colours on the F face (e.g. blue and green), and another pair of opposites on the R face (e.g. orange and red). If you have two pairs of opposites like this, then you have an opposite/diagonal swap on that layer.
If you have an adjacent swap, but can't see the bar, it is easy to confuse this with a diagonal swap. If you can't see a bar, you have to look for opposites again. There will be one pair of opposites at least. Look at the other two stickers. If on the other face there isn't another pair of opposites, we know it isn't a diagonal swap, so we must have an adjacent swap. The two pieces that are swapped are the ones with the opposite colours on them; the bar is opposite that side.