If you're planning to demolish an old barn, possibly because it has become unstable and unsafe or possibly because you need the acreage for another purpose, you might be faced with a requirement to document the barn, its materials and its age to satisfy a local preservation ordinance. Even if you are not planning to demolish the barn and are simply doing research into local history or seeking more information on your own property, determining the age of the wooden beams will tell you much about the structure's history.
Look at the overall construction of the barn and determine whether it was post-and-beam or post-tension construction, or some combination of those--or other--methods. Two key things to remember are that construction methods overlapped and may represent a regional influence or the builder's preferences: not all residences in the High Italianate style are of the later-middle period of the Victorian Era, nor are all residences from the later-middle period of the Victorian Era of the High Italianate style.
Examine any saw and kerf marks visible on the beams; a magnifying glass may be helpful in this endeavor. As any blade cuts through wood, it leaves marks; a pit saw, gash saw and circular saw each leave a very different type of mark.The pit saw left uneven and irregular kerf marks, and was in use from the earliest days of American settlement; the gash saw left large, regular "up and down" kerf marks--it was powered by a water wheel--lumber with this kind of kerf marking is most often found on the East Coast and may date date from as early as the 1600s to as late as the middle 1800s--and the circular saw left an odd circular kerf mark appearing on wood sawn after 1860. and there are even variations within each type of saw. The type and character of these marks often indicate what type of blade was used and perhaps even information about the sawyer and his equipment.
Study any visible nails or screws. Hand forged nails generally date from before 1800; four-sided, with a distinct sharp point, the head was formed by hammer blows that left a distinct pattern shaped like a flower, causing these nails to sometimes be referred to as "rose head nails. Cut nails were cut from a single sheet of material and are three-sided; they date from 1791 and are still available today, as a special order. Wire nails--still commonly used today--appeared in about 1900. Screws made their appearance about 1720, were hand made and usually very small. The machine-made screw we know today appeared in about 1860.
Understand that it's not an exact science: The tone and patina of wood can be telling as to age, but may also be misleading. Like all organic material, wood darkens as it ages. However, if wood has been exposed, even indirectly to smoke or heat (from a kerosene lantern, for example), it will darken just as readily, although the darkening will be more likely be localized in something as large as a beam in a barn. Usually, determining age by tone or patina alone require the services of an expert.
Consider hiring a professional, to obtain even more accuracy with the use of radiocarbon dating--the carbon-14 method--which requires specialized training and equipment to date organic samples such as wood, although there are laboratories which will date a wood sample for a price. However, this method reveals only when the tree, from which the beam was cut, died. In early America, it was not unusual to use standing dead timber for construction, since the wood was likely to be less prone to rot, lighter to transport and easier to cut.