No, there was a thread yesterday about how they are sleepers and blend in as normal neighbors. To me, testing the milk isn't going to do any good. Unless, it is stuff that will kill you instantly, why would they care. They are going to die anyway.
This occured immediately to me too, but I felt foolish stating the obvious. Perhaps it's not so obvious; or perhaps the separate explosive ingredients induce effects tolerable for a couple of hours.
In any event, I would be on the lookout for passengers who get inexplicably sick on any flight.