You would say "Did you get it?" because they "just" guessed. Even though it may have been only a few seconds since they guessed, it is still in the near past. So your question should be in the past: "Did you get it?"
You use "Do you get it?" when you are explaining something to someone. Also when telling a joke.
Why did the chicken cross the road? To get to the other side.... Do you get it?