There are multiple factors that help to determine the age of furniture. Being able to recognize these features takes time and research. An item is generally considered an antique if it's more than 100 years old.
Check for tool marks
Examine the back, sides and bottom of the piece for cuts and nicks. Circular saws and machinery were not available until 1860, so marks from tools like chisels, drawknives, spokeshaves or planes are a sign that the furniture may have been made before 1860. Circular marks from a saw mean that you have a post-1860 piece.
Look at the dovetails
Inspect the dovetails in the joints, especially inside any drawers. If the dovetails are sparse and uneven, it was likely made by hand. Precise cuts and spacing usually means it was made by machine after 1860.
Look for symmetrical differences
Look for variances in designs, patterns and carvings. If aligned aspects of the furniture, such as the legs or feet, aren't exactly symmetrical, it was probably made by hand.
Consider the finish
Varnish and lacquer didn't exist until the mid-1800s. Pre-1860 furniture is usually finished with shellac.
Determine the wood type
The wood used for furniture does vary widely, but knowing which woods were popular at a given time and in a particular area helps to date furniture. Pre-1700 furniture was usually made out of oak wood. Walnut and mahogany became common in the early 18th century. Pine wood has been consistently favored for American furniture.