First get an orange for this trick to work and make sure it is ripe or you can ask a spectator for one. then grab it off them and turn it to the part where the stem used to be. if the little green part is still on the orange take it off.
If perfected you can almost see the little segments that are shaped like the actual segments inside the orange, look at the segments on top and count how many there are.
Then tell someone to split the orange in half, it's better if done at home. then tell them to count the segments and if they were accurate, you are a true mentalist.