static void Main(string[] args)
        {
            var sid = new SecurityIdentifier(WellKnownSidType.NetworkServiceSid, null);
            var hasWritePermissions = HasFolderWritePermission(@"C:\Temp\TestDir", sid);
        }

        public static bool HasFolderWritePermission(string destDir, SecurityIdentifier sid)
        {
            if (string.IsNullOrEmpty(destDir) || !Directory.Exists(destDir)) return false;
            try
            {
                var security = Directory.GetAccessControl(destDir);

                var query =
                    from AuthorizationRule rule in security.GetAccessRules(true, true, typeof(SecurityIdentifier))
                    where rule.IdentityReference == sid
                    select (FileSystemAccessRule)rule
                    into rights
                    where rights.FileSystemRights == (rights.FileSystemRights | FileSystemRights.Modify)
                    select rights;

                var list = query.ToList();

                if (list.Any(rights => rights.AccessControlType == AccessControlType.Deny)) return false;

                return list.Count > 0;
            }
            catch
            {
                return false;
            }
        }

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Security.Principal;

namespace ConsoleApp4
{
    class Program
    {
        private enum MULTIPLE_TRUSTEE_OPERATION
        {
            NO_MULTIPLE_TRUSTEE,
            TRUSTEE_IS_IMPERSONATE
        }

        private enum TRUSTEE_FORM
        {
            TRUSTEE_IS_SID,
            TRUSTEE_IS_NAME,
            TRUSTEE_BAD_FORM,
            TRUSTEE_IS_OBJECTS_AND_SID,
            TRUSTEE_IS_OBJECTS_AND_NAME
        }

        private enum TRUSTEE_TYPE
        {
            TRUSTEE_IS_UNKNOWN,
            TRUSTEE_IS_USER,
            TRUSTEE_IS_GROUP,
            TRUSTEE_IS_DOMAIN,
            TRUSTEE_IS_ALIAS,
            TRUSTEE_IS_WELL_KNOWN_GROUP,
            TRUSTEE_IS_DELETED,
            TRUSTEE_IS_INVALID,
            TRUSTEE_IS_COMPUTER
        }

        //Platform independent (32 & 64 bit) - use Pack = 0 for both platforms. IntPtr works as well.
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Pack = 0)]
        private struct TRUSTEE
        {
            public IntPtr pMultipleTrustee;
            public MULTIPLE_TRUSTEE_OPERATION MultipleTrusteeOperation;
            public TRUSTEE_FORM TrusteeForm;
            public TRUSTEE_TYPE TrusteeType;
            public IntPtr ptstrName;
        }

        enum SE_OBJECT_TYPE
        {
            SE_UNKNOWN_OBJECT_TYPE = 0,
            SE_FILE_OBJECT,
            SE_SERVICE,
            SE_PRINTER,
            SE_REGISTRY_KEY,
            SE_LMSHARE,
            SE_KERNEL_OBJECT,
            SE_WINDOW_OBJECT,
            SE_DS_OBJECT,
            SE_DS_OBJECT_ALL,
            SE_PROVIDER_DEFINED_OBJECT,
            SE_WMIGUID_OBJECT,
            SE_REGISTRY_WOW64_32KEY
        }

        [Flags]
        enum SECURITY_INFORMATION : uint
        {
            OWNER_SECURITY_INFORMATION = 0x00000001,
            GROUP_SECURITY_INFORMATION = 0x00000002,
            DACL_SECURITY_INFORMATION = 0x00000004,
            SACL_SECURITY_INFORMATION = 0x00000008,
            UNPROTECTED_SACL_SECURITY_INFORMATION = 0x10000000,
            UNPROTECTED_DACL_SECURITY_INFORMATION = 0x20000000,
            PROTECTED_SACL_SECURITY_INFORMATION = 0x40000000,
            PROTECTED_DACL_SECURITY_INFORMATION = 0x80000000
        }

        [Flags]
        public enum ACCESS_MASK : uint
        {
            DELETE = 0x00010000,
            READ_CONTROL = 0x00020000,
            WRITE_DAC = 0x00040000,
            WRITE_OWNER = 0x00080000,
            SYNCHRONIZE = 0x00100000,

            STANDARD_RIGHTS_REQUIRED = 0x000F0000,

            STANDARD_RIGHTS_READ = 0x00020000,
            STANDARD_RIGHTS_WRITE = 0x00020000,
            STANDARD_RIGHTS_EXECUTE = 0x00020000,

            STANDARD_RIGHTS_ALL = 0x001F0000,

            SPECIFIC_RIGHTS_ALL = 0x0000FFFF,

            ACCESS_SYSTEM_SECURITY = 0x01000000,

            MAXIMUM_ALLOWED = 0x02000000,

            GENERIC_READ = 0x80000000,
            GENERIC_WRITE = 0x40000000,
            GENERIC_EXECUTE = 0x20000000,
            GENERIC_ALL = 0x10000000,

            DESKTOP_READOBJECTS = 0x00000001,
            DESKTOP_CREATEWINDOW = 0x00000002,
            DESKTOP_CREATEMENU = 0x00000004,
            DESKTOP_HOOKCONTROL = 0x00000008,
            DESKTOP_JOURNALRECORD = 0x00000010,
            DESKTOP_JOURNALPLAYBACK = 0x00000020,
            DESKTOP_ENUMERATE = 0x00000040,
            DESKTOP_WRITEOBJECTS = 0x00000080,
            DESKTOP_SWITCHDESKTOP = 0x00000100,

            WINSTA_ENUMDESKTOPS = 0x00000001,
            WINSTA_READATTRIBUTES = 0x00000002,
            WINSTA_ACCESSCLIPBOARD = 0x00000004,
            WINSTA_CREATEDESKTOP = 0x00000008,
            WINSTA_WRITEATTRIBUTES = 0x00000010,
            WINSTA_ACCESSGLOBALATOMS = 0x00000020,
            WINSTA_EXITWINDOWS = 0x00000040,
            WINSTA_ENUMERATE = 0x00000100,
            WINSTA_READSCREEN = 0x00000200,

            WINSTA_ALL_ACCESS = 0x0000037F
        }

        [DllImport("advapi32.dll", SetLastError = true)]
        static extern uint GetEffectiveRightsFromAcl(IntPtr pDacl, ref TRUSTEE pTrustee, ref int pAccessRights);

        [DllImport("advapi32.dll", CharSet = CharSet.Auto)]
        static extern uint GetNamedSecurityInfo(
            string pObjectName,
            SE_OBJECT_TYPE ObjectType,
            SECURITY_INFORMATION SecurityInfo,
            out IntPtr pSidOwner,
            out IntPtr pSidGroup,
            out IntPtr pDacl,
            out IntPtr pSacl,
            out IntPtr pSecurityDescriptor);

        [DllImport("advapi32.dll", SetLastError = true)]
        private static extern void BuildTrusteeWithSid(
            ref TRUSTEE pTrustee,
            byte[] sid
        );

        static void Main(string[] args)
        {
            var sid = new SecurityIdentifier(WellKnownSidType.NetworkServiceSid, null);

            IntPtr pDacl;
            IntPtr pSecurityDescriptor;
            IntPtr pSidOwner = IntPtr.Zero;
            IntPtr pSidGroup = IntPtr.Zero;
            IntPtr pSacl = IntPtr.Zero;

            var result = GetNamedSecurityInfo(@"D:\Temp\TestDir", SE_OBJECT_TYPE.SE_FILE_OBJECT, SECURITY_INFORMATION.DACL_SECURITY_INFORMATION, 
                out pSidOwner, out pSidGroup, out pDacl, out pSacl, out pSecurityDescriptor);

            if (result != 0)
            {
                throw new Win32Exception((int)result);
            }

            var trustee = new TRUSTEE();
            var accessRights = 0;

            var sidBytes = new byte[sid.BinaryLength];
            sid.GetBinaryForm(sidBytes, 0);

            BuildTrusteeWithSid(ref trustee, sidBytes);

            result = GetEffectiveRightsFromAcl(pDacl, ref trustee, ref accessRights);

            if (result != 0)
            {
                throw new Win32Exception((int)result);
            }

            var accessMask = (ACCESS_MASK) accessRights;
        }
    }
}