It depends on who is doing the assessment and which diagnostic tests they use so it's impossible to say.
That's not a bad thing though, because if everyone knew in detail exactly what they were going to be asked, it would be possible to look up all the questions and answers in advance and maybe skew the results, or at least make the results less defined, which helps no-one. Will it be done by one person or a team of professionals? Would it help her to know in advance if she'd be seeing one person or several? Hopefully they are experienced in diagnosing autism in girls and women. If so, perhaps you can find out and let her know, if you feel she may be reassured by that.
Do reassure her she will be assessed by professionals who are not only very capable of spotting masking, but who also will spot several things neither she nor you have noticed before.
Sometimes you as parent are asked to fill in questionnaires in advance, usually about how your DD presented in the early years, developmental milestones, sensory stuff etc. Sometimes you are asked similar things on the day.
Your DD will probably think a lot of the assessment is an informal chat, or maybe having fun doing simple puzzles, describing what's going on in a picture, but the professional 'chatting' to her whilst seeming to be very informal will be observing her reactions and demeanour as well as listening literally to what she's saying and how she's saying it.
They are trained professionals, they can see beyond masking and beyond what everyone else thinks is 'completely ordinary' behaviour or responses.
Do tell her to be herself on the day, relax as much as she can and understand she can't give 'wrong' answers to their questions.