I read about how the Third Reich had certain factory employees as uniformed police for the factories. But nobody has said who watched the farms, which used the most foreign workers, voluntary or not.

If you mean 'were troops or police assigned to guard them', no. Anyone who decided to disappear would be in an unfriendly country, have no papers and no money and generally find it hard to stay at large. So during the war, there was no point. Forced labour at factories lived in guarded barracks and would be herded to the factory. Those assigned to farm work were in rather smaller groups so they lived on the farm (often in a barn, apparently).

Not quite sure what you mean by "...who watched the farms,..." are you referring to the security? In fact, I believe the Reich used coordinated foreign labor for planting and harvest, which would only have been done a couple of times a year. I've been reading the book "Hitler's Rockect Solidiers" which has some interesting notes not just on the V2 program, but on the lives of individual soldiers engaged in developing, launching or providing security to the V2s-- in each case the soldiers stated they worked on farms as laborers when not otherwise engaged in military duties, so this appears to have been the course for Germans throughout the war. Given the organization of the German Workers Organization, the RAD, prior to 1942, there were apparently many youth laborers available, post 1942, most young men were apparently drafted into the armed forces, but continued to work on the farms when not serving in combat. So, I'm not sure "security forces" were really necessary for farm workers. In factories however, it was a different situation, as the Germans employed skilled foreign laborers and in many cases concentration camp workers with skills, as inthe Mittelwerk V2 assembly factory-- and the SS was brutally in charge of security. However, factory work was a 24/7 operation, whereas farm work is seasonal. Not sure if this answers your question or not, but it does provide some insight into what the differences were. By the way, I served in S. Korea a total of 4 years-- the ROK Army has a similar work program during the rice planting season, and it's not unusual to see ROKA troops assisting farmers with planting-- and US troops are sometimes invited to join in-- I've had the opportunity myself to wade into a rice paddy ankle deep in water and shove a young rice plant into the muddy bottom-- its back breaking work-- my hat is off to those farmers who do it every year.
VR, Russ

At the end of the war, there was no one farmning in Germany, they were all starving.

Some farms at least were working, so some food came from them.