Don't ask a question if you don't really mean it.
When a presenter has a question-and-answer period, they're probably not doing it for the sole purpose of determining who seems like they were listening more than anyone else. If the presenter simply wants to test people and see who was seemingly paying more attention (though questions are not necessarily correlation with attention), then they're not respecting those people who have real questions.
Respect goes the other way, though. If you ask a question for the sake of asking a question, you could take time away from someone else who genuinely needs a clarification, and if you're all crunched for time, that's not good.1 You've taken away someone else's opportunity. For that reason, I can't recommend asking a question if you don't actually have one.
There are other ways to stand out.
It seems that you're really just concerned with standing out. In that case, you don't have to ask a question at the end. The presenter may have left a few things incomplete along the way; asking for him or her to go over them can be truly helpful to the group.
I've done these before. Most arose in cases where I took notes (for academic purposes, if it was a class, or for personal curiosity). Notes let you look back and try to understand the presentation when all of this is over and done. That said, don't take notes if you can't pay attention at the same time! I once asked a decently stupid question because I had missed something the presenter said - because I was overzealously taking notes.
Some things you can do:
- Did they skip a slide for reasons of time? Ask if they can go over to it. I've seen this happen in science presentations when the presenter didn't have time to explain a semi-related part of their work.
- Are you unconvinced by an argument? Ask them to repeat it. If the presenter is trying to convince you of something (here I'm reminded of my philosophy class this past year, where we had to convince each other of every point), they need a strong argument.
- Did they not elaborate on a facet or their presentation? Ask for them to explain it in more detail.
The above options are likely going to be helpful in some way to others. If the presenter's presentation isn't quite complete, your coworkers may also be wondering what was missed. Don't try and nitpick every little part of the presentation, but keep an eye out for major points of clarification.
Take notes as you go.
Recognize these chances when they come up. You're not going to be able to remember these ideas in a flash during a Q+A session at the end, and you run the risk of looking extremely stupid if you mess up (I mentioned that I made that error once). Write things down as the presenter goes over them. Take notes - in moderation.
Again, only ask this if you do want to ask questions. This applies to all questions, not just the sort of inquiries I gave above.
Can you talk to the presenter at the end?
If you want to stand out, this is an excellent way to do it. My college's physics and math departments do weekly or biweekly colloquia, where an outside speaker talks. I've had the chance before to talk to the presenter before, and it's pretty cool. In one case, the presenter was a candidate for an assistant professor. Whether or not they'll get the job is up in the air, but if they do, I'll have made a connection in the department with someone who studies something I think is awesome.
Obviously, don't be a nag or go up to the person when they're busy. I happened to just stop by to thank the presenter in this final case, and we got into a small conversation, which was cool - and lucky.
1 A friend of mine was in a related situation recently. In a large college class, a group of students was essentially monopolizing the question period by asking questions that demonstrated very little effort or critical thinking. They were essentially parroting back information from the professor, with some slight twists, and they were clearly doing this for the attention.
The rest of the class, quite simply, was annoyed. This group had turned the discussion in a completely unproductive direction when something constructive could have been gotten out of the question session. Keep this in mind; the questions you ask can affect the questions others have, and may derail more sincere discussions.