The rule of thumb is, that a skewer (or cocktail stick) stuck into the centre of the cake should come out clean.

Other indicators are that the cake will start to slightly come away from the sides of the tin, and if you gently press the centre of the cake with your fingers then it will rebound (ie. it will feel like a cake).

If your cake isn't cooked, but is starting to burn on the top you can still try to rescue it. Cover the cake with tin foil to try and prevent any further burning, reduce the temperature a little (or move the cake lower in the oven. Once the cake is cooked and out of the oven, wait 10 minutes for it to cool slightly and chop off the burned bits (even the best of us get distracted sometimes and leave a cake in too long!)

If your cake is cooking too fast, make sure that you're using your oven correctly. Different types of ovens require different treatments. Fan ovens need to have the temperature that the recipe specified reduced by about 20% whereas when using gas and conventional ovens you need to make sure that your cake is in the middle - too close to the top and the temperature will be too hot, too close to the bottom and it'll be too cold.

If you forget and dry out your cake by cooking it too long then you can rescue it by turning it into truffles or tiramisu - these are also great ways to use up any offcuts of cake, or stale, uneaten (in that rare circumstance!) cake.