Fun fact: this Apple Support document is just flat-out wrong. It says: "you can identify [a fast charging] cable from other chargers by the USB-C connector and the aluminum around the magnetic charger", but the very short 0.3m USB-C cable Apple started selling in 2018 and has since discontinued meets these criteria and, wouldn't you know it—can't fast charge an Apple Watch.
As someone who travels light, I figured I'd bring my 0.3m cable if I could. Because the Internet provided zero help here, I figured I'd feed the web to mention that as of 2023, the only Apple Watch chargers that support fast charging are 1 meter long. Oh well.