My understanding is that scientists believed it was tidally locked for quite some time (until the mid-1960s, and not confirmed until Mariner 10 in 1974-5): the resonance between its rotation and its orbit (together with limitations of where in its orbit Earth had a good view of its surface) meant that observers routinely saw the same surface features in the same places. Some of that history is discussed here: https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_...