How do I know if my baby's latching on correctly?

The nipple should point to the baby's upper palate and the whole of the areola is in baby's mouth. If the whole areola cannot fit, at least most of it that's below the nipple should be in baby's mouth. It's the lower jaw that "powers" sucking so it's that area with the most friction. You'll want baby's lower part of the mouth to be scraping against the flesh outside your areola (that's why you want as much of the areola inside baby's mouth). Lips should be turned out (like a fish) not turned in (like an old man).