It’s my standard practice to identify each layer with a graphic text object showing the layer’s identity. I place this outside the board outline but near the working area - usually below, or on the left side, of the working area. Once upon a time many of the board fabricators asked you to do this as a way to reduce the possibility of errors or confusion when the layer was displayed in a Gerber viewer.
p.s. - And I always use the “Plot” feature to make PDF’s. Back when I started using KiCAD there was a problem getting decent images with “Print”, but no problem with “Plot”. I don’t recall the symptoms, and it may have been a faulty printer driver rather than KiCAD itself, but I’ve stuck with “Plot” ever since.
@MarcoHess
what I did in OrCAD is to place a layer marker (something very similar to what @dchisholm suggested) which is a standard item in OrCAD where automatically a number in the same copper layer of the layer itself will be added…
the result is something view-able through the pcb board and present on each gerber and printing layers…
unfortunately there is nothing similar in kicad as far as I know…
so I ended up adding a number to each layer with a non routing zone around all the numbered area
Layer Marker seeing from top pcb board: