Savage & Polite's Antique Clock Identification and Price Guide recommends using an appraiser to correctly identify the maker, time period and value of antique clocks. The National Association of Watch & Clock Collectors maintains an online searchable database of websites and research materials to aid in the identification and valuation of clocks and timepieces, as well as information on appraisal services.
Traditionally, an antique clock is considered to be any clock that is at least 100 years old. Mantel clocks from the 1920s and art-deco clocks from the 1930s also meet the modern definition of "antique," though such newer clocks might better be described as "vintage" rather than "antique."
In addition, many modern mantel-style clocks are manufactured in a way that replicates antique designs. Wood clock movement pieces were generally replaced by metal pieces by the mid-1800s, so the presence of wood moving parts can be a good indicator of clock age.
There are distinctions in the antique world between "new antique clocks" and "old antique clocks" made before 1750. Finding the maker's mark on the timepiece can be helpful, but this mark is often on a paper label attached to the clock, and as such is subject to counterfeit efforts.