I only ate commercially made citrus marmalades before and from this experience know that there may be variations in thickness, color and taste.
The kumquat marmalade I made came out thicker than desired, but I'm not sure whether it's due to fruit qualities or mistakes made in the process (the recipe called for 20min of simmering, I needed 1h). And it will become even thicker while in storage.
Are there signs that can tell that it's definitely overcooking rather than ingredients?
Yes, you can cook your marmalade to a reliable consistency. Unlike other types of jam, citrus marmalade is made with a candy-making technique, so you have to stop cooking at the desired consistency.
The easy way to do it is with a candy thermometer. Cook it to 105 C final temperature (220 F), and immediately remove from the heat. It is best to make sure to use a thin-walled vessel, to make sure there isn't unwanted carryover.
Alternatively, you can do a standard gelling test. You need to prepare very cold saucers (freezer is better than fridge) and when you think you're close, you start putting drops of marmalade on the saucer and see how they behave when cooling. Since 105 is a very low temperature for a candy, you won't find any descriptive terms on candy charts, so you have to learn the proper on-saucer consistency without linguistic help. But obviously, as long as your drops are spreading easily, the marmalade is not yet ready. You need to see signs of setting.
