Like other men with a passion for watches, Cade Mlodinoff began his obsession by coveting one model. It was a Tag Heuer. It cost him $400 when he bought it at age 10.

“At the time, it was every dime I had,” said Mr. Mlodinoff, 33. “My obsession kind of grew from that.”

For decades, watches have been symbols of luxury, wealth and personal taste. But their potential as investments is now more easily tracked and understood. You can credit technology for shining a light in dusty watch shops.

An auction for vintage and modern watches this week at Sotheby’s raised more than $9 million, while Christie’s plans to hold an auction of rare watches in Hong Kong on Monday. Last year, Paul Newman’s Rolex Daytona sold for $15.5 million, not including the auction premium.