You need to find out how questions are going to be managed at your presentation. For some presentations it will be quite clear, for example, they might state that in your fifteen minute presentation, you present for ten minutes and allow five minutes for questions. In other cases it might be up to you. Sometimes the convenor or Master of Ceremonies will manage the question time for you. Sometimes people might want to ask questions as you go through the presentation.
Check all this out beforehand. Contact the person organising the event and find out what the expectations are. If you are a novice or nervous presenter it is probably best to keep the questions till the end of the presentation, otherwise you may get a bit lost with your content or the timing. If the format is that you present for ten minutes and allow five minutes for interaction, this is probably enough time for three or four questions. If the talk is longer, say 45 minutes, then you would probably allow 10 to 15 minutes for questions.
For some speakers a bigger fear than difficult questions is getting no questions at all. Then there’s that horrible silence which seems to drag on forever. So here are some ways to invite questions. As you get to the end of your talk you say “In a moment there is some time for questions. Before that I will just recap some of the major points.” And then you highlight some of the areas where there might be questions or you might like people to ask questions. If there is still a silence you can create your own question. For example you could say “One of the things people regularly want to know is [insert question that you know the answer to]”. This can sometimes get the ball rolling.
Another strategy is to place a confederate or stooge in the audience. You give them a question beforehand. (Obviously one that you know the answer to!) You can do the same thing with the convenor. You would say to him/her “When I finish, if there aren’t any questions please ask this question”.
If there are no questions, and this often happens, then say “I think that’s it then. Thank you for your attention” and leave. Remember there may be several reasons people don’t have questions. They want to move on to the next talk, or they have somewhere to go or you’ve answered all the obvious questions.