1839: Look at the curl depicted by Arrow A. If the final curl points to the back of Victoria's neck, it is the First Head. If it points away, it is the Second Head.
1867: Again, look at the curl pointed to by Arrow A. If the Curl is 'loose' (more space inbetween the curls) then it is the Third Head, if it is 'tight' then it is the Second Head. Also look at the hair behind the ear pointed to by Arrow B. If the shape of the hair is more triangular, then it is the Second Head, if it forms more of a rectangle or trapezoid, then it is the Third Head.
1878 and 1879: Look at the hair below the band behind the ear. On the Fourth Head, this hair is much shorter and does not extend as far down.