I have been growing some cuttings of Salvias and other plants. The only way of knowing if there is enough root growth, to pot them up, is to turn the pot of cuttings upside down, remove the pot, and inspect.

If there is good root growth - there is no problem. But if there in no root growth, you want to put the cuttings back in their pot. But you can't. Without the roots the compost has crumbled away and fallen to pieces. So I have put some of my cuttings into transparent pots (just disposable picnic tumblers - with a few drain holes), so that you can see the root growth.

But is light detremental to root growth? On Googling, there are people who say it is, but without an explanation as to why. But at the same time cuttings grown in water have their roots fully exposed to light and don't seem to suffer.