One of the most important steps to making mashed potatoes is knowing when they’re done. An easy, foolproof test is to use a thin wooden skewer. If it enters the core of the potato with little resistance, the potatoes are done.
The potatoes are cooked when you don’t feel any resistance when you poke them with a skewer or a knife. Err on the side of overcooking, which can be remedied, rather than undercooking, which can’t.