How accurately could late 50s - early 60s humans have mapped the solar system?
I recall hearing Professor Lovelock about 10 years ago, describing a discussion with his students in the 1960s - what observation would indicate life on another planet? They concluded that spectroscopic analysis would show that the chemistry of the planet was inconsistent with the planet's age. A slower than expected rate of entropy would indicate the presence of life, because the chemistry of Earth life uses photosynthesis to build giant molecules which store energy by absorbing solar radiation
Spectroscopic analysis was used to examine of the atmosphere of Mars in the 19th century, many decades earlier than the context of your question. Before the space race began, scientists had already seen enough to rule out the possibility of life on other planets in the solar system
Lovelock's discussions about extra-terrestrial life led to his Gaia hypothesis, a set of models which demonstrate that Earth's current atmosphere and ocean chemistry emerged from billions of years of life on Earth