A placement test seems like a good idea, just to help determine which class she belongs in. Do you have an idea of how much of the elementary school curriculum she has covered already? Many schools and school boards have the curriculum for each grade available on line or in paper copy. Whether she is gifted or simply because her homeschooling interests have led her to investigate certain topics at a different time or faster or more in depth than set out by the school, she may have learned some or all of the 5th grade curriculum already. Unless, perhaps, you have been following your school's curriculum closely or using age/grade appropriate off-the-shelf curricula, and you know that's the right grade level for her at this time?
I would start with reviewing the school's curriculum guidelines to get a feel for where she would place. That really only tells you her achievement level. If she's gifted, there's also the issue of her ability level, and what kind of learning environment suits her. If a gifted program is available and she meets the admission criteria, it may be a better placement. The school may require IQ testing for admission to a special program. You'll have to ask what programs are available and how to access them.
If she isn't gifted, but has covered much of the 5th grade curriculum by homeschooling, then it would be helpful to discuss the most appropriate placement - repeat 5th grade (which could be excruciatingly boring) or move on to 6th grade, where she will be younger than the other students.
I don't think you will come across as pushy if you approach the school administrators with a view to finding out what is the best placement for your dd. Explain that you think an assessment of her achievements and ability would be helpful, if the terms "placement testing" and "IQ testing" sound pushy. I think both sound reasonable, though, and a good idea, especially if you are not sure that the regular 5th grade class is the right place for her.