Ensure the needle still finds North. A "dead" compass can sometimes be re-magnetized, but a cracked jewel bearing is much harder to fix.

Larger, often housed in wood boxes or brass gimbals (swivel mounts) to stay level on a tossing ship.

Look for a maker's name (e.g., Stanley London , T. Cooke , or Short & Mason ) and a serial number. "London 1917" stamped into shiny brass is a common red flag for modern fakes.