There are a couple of ways you can check this. The easiest way is to right click on the Computer\This PC icon and choose properties. Then on the bottom there will be a section that says Windows activation. Under that it will show if your copy of Windows is activated or not and give you a product key. If you need to change your key for any reason you can click on Change product key.
Or you can type slmgr.vbs -dli from a command prompt and it will pop up a dialog box with the activation status of your computer.