You can check whether current user belongs to Local Admin Group like this
1. Get reference to current WindowsIdentity
WindowsIdentity wi = WindowsIdentity.GetCurrent();
2. Create WindowsPrincipal using current windows identity
WindowsPrincipal wp = new WindowsPrincipal(wi);
3. Check if user is in "BUILTIN\Administrators" role
bool isAdmin = wp.IsInRole("BUILTIN\Administrators");