It can be a bit tricky to know when crullers are cooked all the way through. Color is the most important indicator - they should be a deep golden brown on all sides. Another thing you can do is use the first doughnut to test the cook time. Cook one cruller in the hot oil, making sure to keep the temperature of the oil as constant as possible and timing how long it takes to achieve a rich golden brown color. Remove the cruller from the oil and watch it for a minute or two. If it collapses on itself after it cools, that means the inside isn't quite done. Simply increase the cooking time for the rest of the batch.