sf::RenderWindow::GetWidth() and sf::RenderWindow::GetHeight() should cut it ?
float OriginalWidth = 800;
float OriginalHeight = 600;
Scale window to 1024x768;
float ScaleX = WindowWidth/OriginalWidth;
if(MouseX == PositionX*ScaleX)
Havent tested this at all, just wrote it as i was thinking at the moment. Something like that should work tho