Provided you have Administrator privileges, you can use a simple WMI class to check whether someone is accessing your resources via the network:
PS> Get-WmiObject -Class Win32_ServerConnection | Select-Object -Property ComputerName, ConnectionID, UserName, ShareName
The same can also be done remotely: add the -ComputerName parameter to Get-WmiObject to see who is accessing the shares on that remote machine. Remote access requires Administrator privileges on the target machine.
