Digital artwork, airbrush techniques, and refined graphics are a hallmark of album covers from 1967 and beyond . This is said to be a direct relation of the psychedelic and drug cultures' influence. Cover content changes as standards of decency shift . Rock and roll and Popular genres in the early years almost always featured very clean-cut looking groups of people, mostly white. Classical music really didn't try much in the way of overtly marketing their artists for sex appeal until about the mid-eighties.
Classical artists are harder to pin-point, though hair and clothing styles help.
As technology progresses and the stylus and tone arm became more efficient, the discs become lighter in weight. After 1968, most of the major labels were no longer issuing new LPs in mono. Sometimes albums advertised there can give you a better idea of the general date of the item you have in hand. Indications of phonogram dates are standard by 1976. 1933 - Articles begin to appear in the library literature on how to organize and catalog collections of sound recordings.