- Send someone back.
- That person makes no changes.
- That person comes back and she will still remember the single time line.
In general, so long as changes don't change the fact that she is using time travel, then she 'll remember that time travel was used.
So how does she know what happens when changes are made? The person who remembers tells her afterward. And she has to tell that person what happened in the new timeline. For example, at the end of X-Men: Days of Future Past
Logan was surprised to find out that Jean Grey was alive in that new timeline.
She may not remember using her powers that time in the new timeline, but she can still learn about it from the person who does remember.
Not every change is as significant as the change from Days of Future Past. Part of the issue here is the paradox problem. If she uses her powers to make a change, then the change will obviate the need to use her powers. But if she is using her powers as a test, then she'll still want to do the test in both timelines.
Tests can also be insignificant. Send someone to perform some task, e.g. set a die to have the six on top. Send someone back in time to change the six to a one right after the change. Then go check it after they get back. That wouldn't prevent her from remembering using her powers, because the visible change occurs after the use of powers has stopped.
Even if there is a change, it doesn't matter if the change doesn't change the use of time travel. Make the decision to use time travel before the changed time and stick to it after the change is made. Then Kitty will remember using time travel in both timelines.
Most of this is speculative. We don't see her testing her powers in the movie. But the question was
Therefore how could she possibly
This is how it is possible. Since she does know things about how her powers work in the movie, we can assume that she did something like this. But we don't know how thorough she was or her exact process.