You can be sure that is not too much sun because you would be seeing red edges on all the leaves (all but one of my jade plants have this feature, and the one that lacks it gets shaded from above). The red edge is a good sign, maybe what you should be aiming for during the cooler months, certainly nothing to be concerned about. I would think from the picture that the problem may at least in part be related to insufficient sun actually.
Watering is very much a personal thing and each place has its own rhythm, but I would not water a jade like an Adenium this time of year. The Adeniums are not real excited about drying out for too long this time of year but a jade (especially one that's not in a lot of sun) can go twice as long between waterings. The way you can know for sure is to keep an eye on how much water they slurp up each time (or stick a finger or moisture meter in the soil to get a better idea of where the moisture is at).
We have lots of jades around here (forgiving climate) and big ones in the ground can survive without any supplemental irrigation through our 3 month minimum summer drought. Up to 5 or 6 months bone dry, with 10 inches of rain total in a good year.