Most antique clocks (for sale) will have signature of the clockmaker on them as a sign of authenticity. You can also use the signature to date the clock. By using a good reference book for clock signatures you can match the signature on the clock with the one in the reference book, giving you more context of the date and clockmaker. There are several reference books, such as La Pendule Francaise, Vienna Regulators and Factory clocks and many more. As you start collecting clocks from in a specific style or from a specific country you can look for more specialised reference books.
The location of the signature can very a bit. Often it’s on the dial or backplate. They can either be engraved on brass dials or on they can be painted when the dial itself is already painted. Sometimes signatures are accompanied by trademarks and numbers at the backplate, but not for the older antique clocks.