Is there any case that got checked in application for visa to US

I'm in biology major. In the 6 year study here, I have only been back to china once, which was 5 years ago, before 9/11. during these years, I have been outside of US, only to Canada and Mexico. I'm going to graduate soon and going to start OPT. I would like to apply for a visa to US from Canada now. I'm wondering if there is any case that they will check you in Canada? I don't want to be stucked in Canada for 3 weeks right before my defense. Thanks. Any response is appreciated.