Mimi, when I read the posting I immediately grabbed my trusty Dictionary of Cliches. English speakers must have an urge to use "differentiate from" instead of plain "tell from" when tryinig to make comparisons. At least American speakers like to differentiate from all sorts of things. It is a rather overused expression. Your sentence is correct. You could have said, "Some people find it difficult to tell an American from a Canadian," and the meaning would have been the same. It is not difficult to tell an American from a Canadian. It would be a problem but for the pronunciation of out which is oooout in Canada and that the distance from Minneapolis to Winnipeg is about 500 miles, eh?