X-Ray crystallography is nowadays an accurate and rapid method of determining conformation in the crystal lattice, which conformation usually corresponds to the preferred conformation in solution.
The first serious applications were in triterpenoid chemistry.
Every chemical reaction has a transition state.
Many transition states have a well-defined preferred geometrical requirement.
It does not seem, however, that organic chemists were much worried about barriers to rotation in organic molecules in general at that time because there was no technique available to demonstrate the phenomenon experimentally.