Gently shake the baking pan or individual ramekins to see if the center is set but still jiggles slightly. The custard should have a soft, wobbly texture in the center while the edges are firmer. Avoid overcooking the flan, as it may result in a dry or rubbery texture. Or, insert a toothpick or a small knife into the center of the custard. If it comes out mostly clean, with only a few small, moist crumbs clinging to it, then the flan is likely done. If the toothpick comes out with raw batter clinging to it, it needs more baking time.