I hate that your daughter's teacher said that to her. Even if she did look at the words, maybe she didn't understand that she wasn't supposed to do that. Regardless, her teacher could have communicated the issue to her in a more constructive way.
If it were me, I would probably send an email to the teacher to clarify why she thought your daughter cheated. If she says she saw her actually look at the words, then you should probably talk to your daughter and let her know for future reference that she should not do that. I would reassure her that it's ok to miss words on a pre-test and that would be better than looking at the word list when she's not supposed to. She will have plenty of time to review the words between then and when she has the real test each week.
If the teacher responds that she just didn't believe your daughter could spell those words without cheating, I would be up front and honest and let her know that unless she actually saw her "cheat", you would appreciate it if she wouldn't give your daughter negative feed back like that. If she didn't cheat, that simple statement could completely undermine her confidence in that area.
Teacher's are not perfect of course, so when a teacher says or does something I don't agree with, I often tell my daughter "I'm sorry your teacher told you that. I don't believe that myself and what your teacher should have said is ..." so that my daughter knows how I feel about the subject and I can usually make her feel better about the situation that way without actually having to confront the teacher. Some things do require a note to the teacher though to clarify the issues so I'm never afraid to send one when one is necessary. In this case, it's important for you to know if she did cheat so you can address that with your daughter as well. In the process, you may find that your daughter's teacher isn't that great at communicating effectively with the children in her class and might need to be addressed further up the ladder as well, especially if it looks like it's a habit and not just a poor choice of words on this one occasion.