Basically, it is also possible to read the activation key of Windows 10 with on-board resources, because this is like in the predecessors Win 7 and Win 8.1 in the registry under:
For the Windows 10 Key, just choose:
Computer: LOCALHOST and directly below: HKEY_LOCAL_MACHINE off.
After clicking on: Start Search, the tool starts to search your registry and read the available keys.
You can complete the search completely, or cancel it manually as soon as your Windows 10 Key appears in the list.
See picture: (click to enlarge)