In most cases you just have to know. There are some ortholinear keyboards with some sort of display or indicator and you could program the backlight to show you

In most cases you just have to know. There are some ortholinear keyboards with some sort of display or indicator and you could program the backlight to show your current layer. It sounds like it would be hard to get used to but a few days was all it took for me to be proficient and I type faster now than I did on a conventional keyboard.