The early border between the Soviet and US/UK zones in rural areas was known as a 'Green Border' and was little more than a number of signs with a few barriers.
From 1947 the Soviets reinforced the zonal borders with more road blocks and barbed wire in some locations. However, crossing between zones was still relatively straightforward if you chose your time and location carefully.
The borders between the Soviet/US zone and the Soviet/UK zone in the early days of occupation
The border in Berlin was composed of signs and lines painted onto the road.
The borders between the the various zones in Berlin