The most easy way to test if everybody understood your lecture, is by having a small quiz at the end of it. As simple as it sounds, there are not many options to gather the answers of students in a simple and fast way.
However, almost everyone has a WiFi-enabled device with them these days, being it a laptop, iPad or smartphone, which means that you can easily use these to gather the answers.
And that is exactly what Kahoot.it does.
This site allows you to simply make quizzes, where the answers can be given via the internet. No registration of the participants is required, all a participant has to fill in is a pin and choose a username. After that you can run the quiz.
Between the questions you will see the scores of each player, sparking some extra competition between the players.
On their site GetKahoot.com you can find all the details and try it out for yourself.
Last week we used it during one of the lectures and I can say that it works really well, everybody was in the game in no-time and it was a real fun way to test us. Highly recommended (at least from the user side).