How to tell if someone is remotely accessing your computer windows 10?

Here are some things which might help to answer your question - how to tell if someone is remotely accessing your computer windows 10:

Turn off/Unplug your router, then boot up your computer.

Open settings and go to Apps & Features, look through the list for programs like 'TeamViewer'. I suggest you google search some top remote access programs and try to identify any you might have installed
Uninstall those you find, and also anything else you don't need. If you don't know what something is, Google the name first to find out.
Go back to the main settings window, and go to System > About. In the Related settings list on the right, select System info.

• Click Advanced System settings on the left, and go to the Remote tab
  
• Untick 'Allow Remote Assistance connections to this computer'
  
• Back in the settings main window, open Update & security
  

Ensure both Windows and Windows Defender are up to date, if they aren't reconnected to the internet and update them. Run a windows defender scan. If those steps don't work or you want to be extra sure, you will unfortunately have to Nuke it from orbit, but after doing so I suggest you step back through the above to untick the Windows built in 'Allow Remote Assistance connections to this computer'.