Just the time of the claims and British records of the time (allowing for the one hour time difference).
To be fair to Schopfel, he had a rather busy few minutes and can hardly have watched what happened to the three aircraft he claimed. All three were hit.
It certainly looks like him and I have a picture of what I think is the same aircraft which matches those markings. There are another three victories obscured by the elevator in the picture posted above.
The picture must have been taken in the summer of 1942, probably at Theville in France.