The first experiment would be to determine if Arsenic is present. A quite direct experiment would be to take advantage of the anomalous scattering of As vs. P by using a tunable X-ray source (synchrotron). By using a wavelength of just over 1 Angstrom As will be quite distinct from P.
I would expect the radius and pitch not be drastically influenced. However, three dimensional structural prediction is quite difficult and it feels a bit uncomfortable speculating.
If the number of arsenic atoms incorporated are as few as the molarity calculations indicate (i.e. ~1 per 5kb), I doubt that x-ray experiments would be definitive. And they're hard to do.
At any rate, you'd have to extract and crystallize a fragment of DNA that was particularly enriched for As, which would necessitate most of the other controls that people are suggesting anyway. You'd probably have a pretty good confirmation/refutation of the paper's claims by the time you were set up to shoot x-rays at a crystal.
I would expect the radius and pitch not be drastically influenced. However, three dimensional structural prediction is quite difficult and it feels a bit uncomfortable speculating.