There is no "isFullscreen()" in SFML, but it is easy to make your own:
sf::RenderWindow window(sf::VideoMode(WIDTH, HEIGHT), TITLE, sf::Style::Resize | sf::Style::Close);
bool isFullscreen()
{
sf::Vector2u window_size = window.getSize();
sf::VideoMode videomode = sf::VideoMode::getDesktopMode();
return(
window_size.x == videomode.width &&
window_size.y == videomode.height
);
}
Rob