Generally what I do is take a picture with the app anyway of the location the headstone would have been, that way they can see on the map where the grave is.
Then if burial records exist, I add the information from the burial register into BillionGraves. In the box that says "Grave Information" I add that the details were obtained from the cemetery office/online register etc.
If I plan well before I go to the cemetery I will also take an iPad with the burial register details on it and photograph the screen at the burial location to make sure the details and the photo match up. You could do the same with a piece of paper or cardboard.