Scoop some up with a spoon or spatula and watch what it looks like as it drips back down into the pan. When you stop getting separate drops, but instead they start joining together into more of a stream or sheet, then it's more or less done. (It will also just start looking and feeling thicker and more like syrup.) I suggest taking it off the heat when you first start thinking maybe it's close to being done, letting it cool off, and then seeing if it seems thick enough. It will thicken up quite a bit as it cools. If it's still more runny than you like it even after it's cool, heat it up again and boil it some more.