In canning, there are several ways to tell if your jam has reached the set point or will come to a good set. Temperature is definitely a helpful indicator of doneness as you want most jams to reach and remain at 220° F before filling and processing the jars. You can also run the plates in the freezer test (my least favorite for a number of reasons) and/or the how is it sheeting off the back of my wooden spoon test (a good one!). Experienced canners can also tell when the sound of a spoon run through the hot jam changes to one that alerts you that success is near.
Even with those tests though, some fret that their jam is still too loose: will it ever firm up? This picture of my leftover jam in the pot (the rest was processing in the water bath) shows that this jam will assuredly reach a terrific point of jamminess. Do you see how the jam isn't filling in the open areas? It's just sitting there, the sugars nicely crystalline, the pectin chains all back together again.