I’ve always just used the bead test where you drop a drop of water in a dry pan and if it beads up and rolls around, instead of just sizzles, then the pan is hot enough to add oil (although this also works if it’s too hot, but I have a good sense of how long it takes to get to this temp, so I’m usually testing just before and just after it hits this temp). Then when the oil is shimmering, this is the time to add food.