For example, unsplash.com has an infinite scroll. Let's say we want to change it to pagination. What can be the best questions to ask a user for a usability test?
Pagination allows you to find something later by page it was on. (When you didn't save it, because you didn't think you will need it, you will still vaguely remember which page it was on.) Infinite scroll is more "lazy" - comfortable to use at the moment, but you won't later find anything that you see there.
So do your users like to go back to what they seen on your page or are they using it for a seamless relaxation? You should ask both of this questions.
However A/B test are still a nice option, it can show you better what really works the best for your users. Self reported data are not always reliable and also what works for others can work differently for your site.