Clang

Study catalogs. Often there'll be gaps where the archivist didn't scan the whole thing (they might have only been interested in road bikes and not mountain bikes, kid's bikes, etc). Sometimes there'll be gaps in years. The real use is getting accustomed to how bikes change in spec and appearance as they go from entry level to high-end. Looking at catalogs from many years can teach you how product lines evolve through time. Even if you get familiar with one brand that's prevalent in your area, you'll be able to apply what you've learned to a complete mystery bike.

This is by no means a comprehensive list, just something that covers the bulk of what you'll see for sale in the US.

Bridgestone: https://www.sheldonbrown.com/bridgestone/index.html
Cannondale: https://vintagecannondale.com/catalog/
Centurion: Centurion Catalogs
Fuji: https://classicfuji.posthaven.com/archive
Miyata: https://www.ragandbone.ca/Miyata/miyata_selector.html
Motobecane: https://bulgier.net/pics/bike/Catalogs/Motobecane/
Peugeot: Peugeot Brochures - USA
Raleigh: https://sheldonbrown.com/retroraleighs/index.html
Schwinn 1971-1980: https://waterfordbikes.com/SchwinnCa...inn_1971_1980/
Schwinn 1981-1990: https://waterfordbikes.com/SchwinnCa...990/index.html
Specialized: https://www.retrobike.co.uk/gallery2...ve/Catalogues/
Trek: Vintage Trek Bicycle Catalogs and Bike Brochures, Fisher, Klein LeMond
Trek model by year and colors (ctrl+f for the model number is great quick ref): Trek Bike Models by Year and Color
Univega: https://univegacatalogs.wordpress.com/