Step 1: find the movement serial, not the case number. Case makers numbered their cases in their own sequences, especially in the United States, where cases and movements were usually made by separate firms. Case numbers do not date the movement. The movement serial is stamped on the top plate or a bridge, and you can usually see it once the back or inner dust cover is opened.
Step 2: identify the maker from the movement, not only the dial. The name on the movement plate is the reliable one. Dials were often replaced, and retailers' names appear on dials and even movements. Private-label American watches still carry the factory's serial sequence, so the factory must be identified from the movement's layout. The maker pages and serial guides on this site show the usual signatures.
Step 3: look up that maker's own table. Elgin, Waltham, Hamilton, Illinois, Rockford, Hampden, South Bend and several others have published serial-to-date tables. See the maker-specific serial guides in the serial-number research section. Tables for different makers are not interchangeable: serial 2,000,000 means a different year for every company.
Step 4: read the result as a range. A table date usually marks when that block of serials was started. Movements were then finished, cased and sold over months or years. 'c. 1905' is an honest result. '1905' is usually false precision.
Step 5: check it against other evidence. The calibre, the jewel count, the style of the case and any hallmarks should all fit the serial date. A Swiss or English watch without a serial table can often be dated from its hallmarks (for British silver and gold) or from its case construction.
When there is no table: many Swiss makers' records survive only in company archives. Some firms, such as Patek Philippe, Vacheron Constantin, Longines and Omega, offer paid archive extracts. For unsigned movements, the construction method in 'Dating an Unmarked Movement' is the best route.
Caution: a serial date tells you when the movement was made. It does not show whether the case, dial or hands are original to it. Recased and married watches are common, so record every part separately.