Stick a toothpick or a skewer in a few places along the center of the loaf — it should come out cleanly (or with a bit of raspberry juice, but no batter). If there are still some spots that are undercooked, return it to the oven for a few minutes at a time until it's done. If the top of the berry loaf cake is browning too much for your liking while you're waiting for it to finish cooking, tent it lightly with some foil.