I second letting her run around naked. Either way it won't be a clean catch and they might want to do it again.
My 22 month old is not potty trained but i got him a potty and he sat and peed in it to my surprise. Not sure if he knew what it was from nursery but you could give it a go.
Have her use a potty with a basin so you can collect it
Let her run around without a diaper and watch her like a hawk. She will pee on the floor,but you can get a sample that way. They also could catheterize her. Some labs will do it,but generally the doctor needs to do it.