As your pancake sets, you will see small bubbles forming on its surface. Once you notice bubbles, shake the pan to see if the pancake is loose. Once the pancake is loose, and the edges have cooked a bit, you can be sure it is time to flip. This should only take about a minute at the most.
Pan getting too hot?
Depending your pan, as well as your hob, you may notice that you start out well with your first pancake on medium-high heat, but that the pan quickly gets too hot. If you notice a lot of sizzling or smoking, it's time to turn down the heat to medium! Often starting at medium-high and turning to medium after you flip your first pancake works well.