Lookout for this in the ingredients: You probably know that the largest ingredient (by weight) must be the listed first in the ingredient list. And the second largest ingredient will be listed second and so on. Look for the sugar split into two types of sugar such as cane sugar, sugar, sucrose, corn syrup etc. Since the sugars are split, there appears to be less sugar than the fruit in the jam or product. Separately each sugar weighs less than the fruit, but together they may very well be more.
I will say this is not always the case. You must use your judgement. For example when you are making cookies, you split the sugar into white and brown. This is intentionally done for flavor: not too molassesy, but still sweet and perfectly delicious.
Check the sugar in grams (g) and compare that to the amount of grams in one serving. This will give you a clearer understanding of the percent of sugar to other food you are eating. For example if one serving of jam is 18g and the added sugar content is 17g you can easily see that it is mostly added sugar. Nothing wrong with this necessarily, but we'd like you to make informed choices.
In this example it's 50% sugar, which is split into corn syrup, high fructose corn syrup and sugar.