If you don't often use your phone (or camera) for recording, practice doing it before you go to the airport.

If you'd like to be more subtle, you can just record sound. For example, if your camera records video, you can set it to record (video and audio), but don't worry about where it's pointing (or even that it's all the way out of its case).

But again, it might make sense to try stuff and see what the quality is like, where/how far away the device is likely to be, if you need to boost input or other settings...