Best sources (in order)
1. Markings on the battery you remove.
2. The official manual or support page for that exact model.
3. Our device pages (with links to manufacturer sources).
4. Measuring size and checking the letter code — last resort.
If the old battery is blank
Measure diameter and thickness in millimetres; note any voltage printed in the battery tray.
Search the model on this site, then confirm against the source links.
Watch modules
Casio and similar watches often key off module numbers — two DW-5600 variants can differ (for example CR2016 vs CR1616).