This is extremely easy to do - particularly the farther north one goes (because the extremes of different positions of sunrise are exaggerated). The basis for this isn't knowing the length of the day, but rather it is in observing the farthest reach of the rising and setting of the sun - a similar method can be used for calculating the winter solstice. When observing from a set position, indicated by a pole or some other standing marker, one can trace the shadow of the rising sun. For the summer solstice, the pole's shadow will reach it's most southerly mark.

There is an experiment that is easily replicated. During the last year of my employment I had an office that faced due west. I marked the shadow of the rising sun for the winter solstice, to record its most southerly extreme (the shadow pointing to its most northerly extreme). Since I knew I would be retiring the following winter solstice, I spent the ensuing twelve months watching the rising sun chart its course until it return to the winter solstice. As long as one starts from a given position, it is immediately apparent when the sun has completed its annual cycle.