Scratch the surface of one of the broken nipples.
Silver grey = ally and they will always break if the spoke is too short.
brassy coloured is what you want to see. They will be more tolerant of short spokes. They usually won't snap, but the thread will pull out.
If you're breaking either then it's time for a respoke anyway.
[Edit] Black DT comps come with alloy nipples. All the others come with brass. Only the Swiss know why!