In my state, when you are hired you have to be notified that they may do drug testing. Unless it is a new policy, then they have to notifiy all of the employees in writing, I think it is 30 days prior to any testing.
Where I work we drug test and background check all prospective employees. Unless there is a problem or accusation, that is the only drug test you will be given. But we are a boarding school, so we have to adhere to stricter standards than most.
But the easiest way to find out is to call and ask.