I would contact the school and see if they'll tell you how she is doing. However, given that you are not a parent, they might not give you this information and will say that only parents and specific people who have already been authorized are allowed this information. You won't know for certain unless you contact the school though, good luck.
I would spend time with your granddaughter while she does her homework. Talking, listening, and helping her will tell you most of what you need to know. That would be the first and most important step. I also tutor individual students and that is when they make the most progress.
when she comes to visit you, you can just ask her about her results or else you can also visit her school and talk to her teachers so that you get a feedback on how she is working at school as well as her behavior.