You could watch the app once the car has reached the 80% charge.
I just happened to catch my SR in the act this morning. I got home last night at the back of eleven with 57% battery, put the car in the garage, hooked up the granny charger, and went to bed. At that point the app told me it would be charged at 11.58 am today. I checked in the morning and it was now estimating 11.22 am, which proved to be the case. I looked in on it at that time and actually saw the power input fall from about 1.8 KWh to 0.02 KWh between two scans about a minute apart.
It then puttered along at about 20 watts input (sometimes a bit higher, I have seen 70 watts) for the next half hour. At 11.52 the readout changed to show the car was completely done, no power being drawn at all. So its balancing was complete. It's pretty much always like this, usually 30 minutes but I've seen it go on for 45 minutes.
It's presumably not so easy for people with variable tariffs and charging in the middle of the night, but if you check on the car with the app just after it's due to have reached the 80% target battery level, you should be able to see whether it's balancing or not by whether or not you get a period of low power draw before the app finally shows that the car is no longer charging.