A full sample for obtaining the current system text scale. It doesn't rely on any libraries (such as WRL or C++/WinRT) and uses only base Windows SDK APIs.

TextScaleSample.cpp

#include <windows.h>
#include <roapi.h>

#include <iostream>
#include <cwchar>

// For Windows.UI.ViewManagement.UISettings
#include <windows.ui.viewmanagement.h>

int main() {
    ::RoInitialize(RO_INIT_MULTITHREADED);

    HSTRING_HEADER header;
    HSTRING hstring;
    ::WindowsCreateStringReference(
        RuntimeClass_Windows_UI_ViewManagement_UISettings,
        wcslen(RuntimeClass_Windows_UI_ViewManagement_UISettings),
        &header,
        &hstring);

    std::cout << "Activating UISettings" << std::endl;
    // TODO: Use an RAII container/wrapper (such as Microsoft::WRL::ComPtr from the WRL library) to prevent leaks.
    IInspectable* uiSettingsAsInspectable = nullptr;
    HRESULT hr = ::RoActivateInstance(hstring, &uiSettingsAsInspectable);
    if (FAILED(hr)) {
        std::cout << std::hex << hr << std::endl;
        return 1;
    }

    std::cout << "Querying for the appropriate interface..." << std::endl;
    ::ABI::Windows::UI::ViewManagement::IUISettings2* uiSettings = nullptr;
    hr = uiSettingsAsInspectable->QueryInterface(__uuidof(uiSettings), reinterpret_cast<void**>(&uiSettings));
    if (FAILED(hr)) {
        std::cout << std::hex << hr << std::endl;
        return 2;
    }

    std::cout << "Asking for the current scale factor..." << std::endl;
    double factor = 0;
    hr = uiSettings->get_TextScaleFactor(&factor);
    if (FAILED(hr)) {
        std::cout << std::hex << hr << std::endl;
        return 3;
    }

    std::cout << "Current text scale factor == " << factor << std::endl;

    return 0;
}