Walk over to the person who is texting. Do not look at them. By just coming close to them you will put them in a bit of a spot and eventually they will put their phones in their pockets. This way you can make them listen to what you have to say without making them realize if you saw them texting.
Another way is that you can refer to the person who is texting by calling out his/her name, looking at another direction. This way the individual will briskly put the phone down and listen to what you have to say. Do not put the individual in an awkward position. This way you will lose your credibility and make fun of someone at his/her expense.
Do not let audience know that you have seen him/her using their phone. Make them feel as if you are so involved in your speech that you do not actually care if he/she is texting or not.