To the best of my knowledge, there is no simple way to determine which objects are only being referenced by Cesium objects.
I came across this Cesium community forum post yesterday afternoon. The post discusses freeing memory after unloading 3D Tiles. It seems like @marcel.wendler is also considering memory-related constraints when using 3D Tiles. I thought this may be of interest to you.
Here is another memory-related thread that you may find interesting.
We are happy to have you as part of our community! Feel free to ask/answer as many questions as you wish.
-Sam