My job is demanding. Lots of travel, difficult people, high pressure and high stakes meetings. I have a candidate who is absolutely fantastic on the content. But I'm worried about how she would keep up and handle the pressure, and whether she would burn out quickly.
Ask them scenario based questions. How would you handle a situation where you had less than 24 hours notice for a meeting with your largest client and x happened?
Ask them how do they prioritize their days? If it is too rigid then that may be a clue that the are not flexible to handle emergencies as they arise.
Ask them what their ideal % of travel would be (that can tip you off on what they are actually looking for)
Better than that ask about situations she has already experienced--an example of a high pressure situation. If she says that was running out of stamps, you know she's probably not your girl.
I would ask for examples of situations that have been stressful and how she handled, examples of difficult clients, what she does to manage stress.
What triggered your concern about her ability to handle the pressure? Eg was it an answer to a question or some deficit in her resume? Can you go back to that and ask more probing questions?
I think it might be helpful to ask her for examples of things she's done in the past, rather than how she'd act under a hypothetical. I think it's easier to BS the latter. Surely everyone for any job should be able to give an example of a high stress situation, and how they handled it. If she's stretching for good examples, or seems stressed just thinking about the answer to a question like that, you've got your answer.
You could ask questions about how they handle deadlines. The deadlines at my previous job were set in stone and you couldn't miss them, at my current job they are more flexible. Ask for specific examples of how she met a seemingly impossible deadline or client request.
I'd also be transparent about the environment. My previous job was pretty fast paced and we were upfront about it during the interview process. I don't know how many folks turned down positions because of the environment, but they at least knew what they were getting into.