Hi Lukas,
You can use the update() command to see what scenenode has been selected.
https://www.adobe.io/xd/uxp/develop/plugin-development/plugin-structure/handlers/
It may or may not have changed though. You could do a changed properties check on each update and create your own hash.
If you’re wanting to compare pixel data you might have to wait for a native API to be added. There are external libraries that can do it but they are relatively slow (some notes on the forums here) and it may interfere with the user interaction to run a compare on each update. YMMV