Did the McD's issue come up during routine monitoring of their activities or was the investigation triggered by a complaint? (Maybe I haven't read something).
According to the statements made by the minister the Ministry had responded to allegations brought to their attention by the CBC, and sent inspectors to the restaurant(s) to conduct a on-site investigation(s). I don't know how they determined that what was going on had broken the rules, but I would assume that they probably requested the manger on duty to supply them with all job applications that had been recently received. One look at those applications and confirming the number of foreign workers who were employed there would have likely provided enough proof to terminate the franchisee's ability to bring in additional TFW's.
And sebberry, you're absolutely right regarding crossing the border to work. Border guards will turn you away even if they suspect you could be working and even if its for only one day or several hours. If they have any suspicions whatsoever that an American might be taking work away from a Canadian no matter how short of a duration that work may be they will turn you away, and vice versa. Meanwhile Canada has brought in 300,000 TWF's while 1.3 million Canadians are out of work.