And, what you're wearing, and the lack of cigarettes, and they way you walk, and lean against the counter, and how close you do or don't stand, and a bunch of other clues.

Not necessarily. Depending where they grew up in Canada, they might have a decent background in basic French. Enough to get a decent accent and know their colours, days of the week, greetings, and other simple stuff. I think the provinces close to Quebec/New Brunswick generally have decent French instruction in public school, but BC/AB/SK have terrible French education at most schools.

But ain't no way most English-speaking Canadians have the vocabulary or comprehension speed for any real functional use.