Ooh zombie thread!
I've also been told it's pointless / virtually impossible to swab for sulphites. I have an interesting story to tell about that though. We had sulphited dried apricots which came back negative for sulphites. If you know anything about dried apricots you'll know that in all probability they probably have too much sulphites in them rather than too little. It turns out the lab method for testing sulphites is really, really prone to "losing" the sulphites if you mash up the sample too much. So, they tend to err on the side of caution on this, perhaps too much so. Meaning my sample of bright yellow apricots came back as sulphite free which is impossible.
That cautionary tale aside, what we do is we test a product containing sulphites as a positive control then we do a normal clean then test the first three products made on the same equipment for sulphites to validate the efficacy of the clean. We then verify by visual inspection. Fact is it's quite hard to cross contaminate with sulphites. If you had egg, nuts, mustard etc, I'd use the rapid test kits as well but for sulphites even if the rapid kits exist, I wouldn't bother. If you do want to and can find a kit for rapid swabbing of sulphites, the first thing I'd do is a positive control, i.e. check it works on a "dirty" piece of kit which has been processing sulphites in high concentrations. If it doesn't, then you kit is pointless anyway, either because the sulphites are "lost" during the processing or because the kit don't work!