there is no PRTG sensor which can monitor that directly. You might want to have a look at this custom sensor. It does not detect logins into a domain, but into Windows computers which are added to the PRTG monitoring.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.