They have their own internal policies for this just as in any workplace.
Think of them as the Police, Police don't get breathtested like a civillian whilst they are at work, same for these guys, but of course if they are suspected of activity thent they will be.
Of course if they were flying as part of their job they would be under scrutiny like any other pilot, but think about it, if you were a cop and you were on a random breath testing station and another cop drove by in a marked cal, would you pull him up and test him? I don't think so.
I can imagine their random testing will be blatantly obvious at the airfiled on the day, they may as well put a sign out the front like when they are ramp-checking.