There are a fair number of roads that have nobody looking into your car, although there is a shack where you are supposed to sign in.
lol
According to a report on CFCN, the Calgary CTV affiliate several years ago (I guess that it was soon after 9/11), on the AB/MT border at least, there is apparently CCTV linked to motion detectors on both sides of the line and the border is also monitored by helicopter, mounted patrol and 4x4 patrol. I believe that at that time, the border was also patroled by joint teams of US Border Patrol and RCMP and Canada Customs officers. I don’t know if the process has changed, but the government departments have changed. (Now it is the USCBP and Canada Customs has become the Canada Border Services Agency and those folks are now armed.)
This report also stated that in the east, past where the Great Lakes/St. Lawrence River form the boundary, they also use CCTV and motion sensors where there is a land crossing. I guess the bottom line is that if someone wants to get into Canada from the US (often coming from Central and South America) or into the US from Canada, they will find a way. It seems to me though, that most illegal immigrants to the US come from Mexico.