You could ask indirectly.

For example talk about travel requirements generally like how much time you will spend on the road and then say something like "so what's your personal favourite airline?", "do you find you do much work on the plane or take care of that in the office/hotel?" and "on a long flight do you just put the seat back and sleep, or are you mainly flying in daylight hours?" This will allow them to say things like "we always fly the cheapest" or "I personally love United, especially the new Polaris seat", which you can then interpret appropriately.

Then if you can get an offer, that is the stage to negotiate premium travel formally.