"Fast" is a relative term. The only way to get a specific pixel is through sf::Image::getPixel(). To get an image instance of the window content you could draw to a render texture and then copy the texture content to an image. This procedure is however not very fast, so it really depends what you want to do.
If you just want to manipulate the pixel, you're better off using a shader, which would also be the fastest way.
