Of course the only way to know for sure that fuel at a particular station is UBA is to draw a sample and have it laboratory tested. This is exactly what I have done multiple times, at several Pemex stations in the Northern Baja. I found one sample, almost two years ago, that was higher than 15 ppm, suggesting deliberate or inadvertent contamination with higher sulfur diesel, but since then I have found only 15 ppm ULSD from the USA or Japan.
I do not visit the Mainland any more so I can not sample the fuel for testing, but the reports I get from owners of later model diesel pickups helps me confirm whether ULSD is present in a particular area - no problems reported means ULSD diesel at the pumps. Also, I am getting a very strong feeling that, for the first time in ten years, there are very positive signs that ULSD will soon be all over Mexico.