Reputedly invented by Benjamin Franklin, the rocking chair can be distinguished from other examples of rustic seating such as the Windsor chair by the manner in which it rests upon two curved, ski-like lengths of wood--known as "rockers"--enabling the occupant to shift his body weight into a position of optimum relaxation. Although it can be hard to date examples of such a timeless design with any precision, certain features are strong indications of age.
Look to see how comfortably the chair sits upon its rockers. The earliest rocking chairs were ordinary slat-back kitchen chairs--that is, chairs whose backs consist of a top-rail and a single thick lower rail--attached to rockers almost as an afterthought. If your rocking chair is like this, then it might very well date to before 1830, which was when the "Boston rocker" with curved arms and back was introduced.
Examine the material from which the rocking chair is made. Early examples would have been constructed from solid local timbers. By the 1860s, the popularity of home-grown rocking chairs was being challenged by cheap imported bentwood chairs made by the German industrialist Michael Thonet. Sometimes ebonized, Thonet's bentwood rocking chairs have the appearance of moulded cane-work. Early examples made by Thonet's factory are rather more elaborate than later examples from copycat firms. Non-bentwood chairs from this period were increasingly made from painted pine.
Inspect the rockers to see whether they employ any unusual mechanism. In the 1890 to 1910 era, there was a vogue for mechanical rocking chairs, for example ones where the seat itself rocked while the legs were fixed. Usually upholstered with carpet-style fabrics, these chairs can be dated with confidence to that narrow turn-of-the-century period.
Among early slat-back examples, look out for the two versions of Shaker rocking chair--the so-called "Sister's" and "Brother's" rockers, with the former being much slighter than the latter, while the latter's arms terminate in mushroom-shaped protuberances.
If buying a rocking chair, check the base of the back supports for any cracks or mends resulting from overvigorous rocking.