Check if the process running under an admin user. Tested on Windows 95-Windows 10 (checks for elevation on Vista+)

admin.cpp

#include <Windows.h>

#pragma warning(push)
#pragma warning(disable: 4996)
bool RunningAsAdmin()
{
  OSVERSIONINFOA os_version_information;
  os_version_information.dwOSVersionInfoSize = sizeof(os_version_information);
  GetVersionExA(&os_version_information);

  // Windows 95/98/ME are not multi-user OSs
  if (os_version_information.dwPlatformId == 1)
  {
    return true;
  }

  const HANDLE process = GetCurrentProcess();
  const HANDLE heap = GetProcessHeap();

  HANDLE token;

  bool running_as_admin = false;

  // Pre-Windows Vista, we only need to check if the user in admin group
  if (OpenProcessToken(process, TOKEN_QUERY, &token) != FALSE)
  {
    if (os_version_information.dwMajorVersion < 6)
    {
      DWORD required_size;

      if (GetTokenInformation(token, TokenGroups, NULL, 0, &required_size) == FALSE
        && GetLastError() == ERROR_INSUFFICIENT_BUFFER)
      {
        const DWORD token_groups_size = required_size;

        PTOKEN_GROUPS token_groups = reinterpret_cast<PTOKEN_GROUPS>(
          HeapAlloc(heap, HEAP_GENERATE_EXCEPTIONS, token_groups_size)
        );

        if (GetTokenInformation(token, TokenGroups, token_groups, token_groups_size, &required_size) != FALSE)
        {
          PSID admin_group_sid;

          SID_IDENTIFIER_AUTHORITY authority = SECURITY_NT_AUTHORITY;

          if (AllocateAndInitializeSid(&authority, 2, SECURITY_BUILTIN_DOMAIN_RID, DOMAIN_ALIAS_RID_ADMINS,
                                       0, 0, 0, 0, 0, 0,
                                       &admin_group_sid) != FALSE)
          {
            for (DWORD i = 0; i < token_groups->GroupCount; ++i)
            {
              if (EqualSid(admin_group_sid, token_groups->Groups[i].Sid) != FALSE)
              {
                running_as_admin = true;
                break;
              }
            }

            FreeSid(admin_group_sid);
          }
        }

        HeapFree(heap, 0, token_groups);
      }
    }
      // On Windows Vista onward, being in admin group is not enough , user must run-as admin
    else
    {
      struct
      {
        DWORD TokenIsElevated;
      } token_elevation;

      DWORD return_length;

      if (GetTokenInformation(token,
                              static_cast<TOKEN_INFORMATION_CLASS>(20),
                              &token_elevation,
                              sizeof(token_elevation),
                              &return_length) != FALSE)
      {
        running_as_admin = token_elevation.TokenIsElevated != 0;
      }
    }

    CloseHandle(token);
  }

  return running_as_admin;
}
#pragma warning(pop)