To test for doneness, slip a small plate in the freezer for 10 minutes. Place a small spoonful of marmalade on the plate and let it cool for a minute or two. Gently nudge the marmalade with your fingertip. If it wrinkles or forms a skin that holds its shape, it's done. If it's still runny, continue simmering for a few more minutes and repeat the test.☞ TESTER TIP: Want a more precise test? Cook the marmalade until it reaches a temperature of 217-220°F (103-105°C). That's it.