The add-in you wrote will be written in either Visual Basic or Visual C#.
Thus like any other .NET application your add-in code can query the
System.Security.Principal.WindowsIdentity
and get the current user identity and principal and identify if the user is authenticated and has windows principal in build-in administrator role or not.
Sample code to verify if this Visual studio instance is done using "Run As Administrator" or not.
WindowsIdentity identity = WindowsIdentity.GetCurrent();
WindowsPrincipal principal = new WindowsPrincipal(identity);
bool isAdmin = principal.IsInRole(WindowsBuiltInRole.Administrator);
Above code is in C#