@olivierlambert Here's how VirtualBox does it.
When the VM is the same as snapshot:
After the VM is run and has changed:
It also works when you have multiple different snapshots, or branches of snapshots - it will tell you which snapshot the current state is based on, and whether it has changed.
I guess in XO, you could have "Current State" or "Current State (changed)" next to the snapshot in the snapshots tab. Or icons representing them.