Undeniably, the components you've mentioned, such as paint scheme, decal design, and bottom bracket shell, can be crucial in identifying vintage bikes. To delve further, the type of brakes and derailleurs installed may also provide telling evidence.
For instance, early Campagnolo derailleurs or Mafac brakes could indicate a French or Italian bike, while Simplex or Huret components might suggest a French origin. The materials used in the components, such as aluminum or steel, can also provide clues about the bike's age and manufacturer.
Another aspect to consider is the wheel size and tire width. Vintage bikes often had smaller wheel sizes, such as 26" or 27", compared to modern bikes that typically use 700c wheels. The tire width can also indicate the bike's intended use, such as racing, touring, or commuting.
Lastly, don't overlook the frame geometry and construction. The tube lengths, angles, and diameters can provide insights into the bike's intended use and manufacturer. Additionally, the type of lugs, welds, or brazing used in the frame construction can also provide valuable clues about the bike's origin and age.