To identify a vintage guitar, examine the material the instrument is made of, check if it is hand or machine made, determine if the wood is solid and thick, and note if the wood around the guitar's hardware is darker than the wood in the other areas. Compare the guitar with the pictures given in relevant books and websites.
Vintage guitars are those that were made from 1920 to 1970. Till the 1960s, guitars were commonly crafted out of Brazilian rosewood, which in the later periods was deemed as an illegal import.
If the details on the instrument are identical to one another and seem perfect in design, it indicates that the piece is machine-made. Guitars with individual and unique detailing, and with irregularities on the material are hand-made.
Also, guitars with wood that is slick or thin are more modern than those with thick wood. Darker wood around the guitar's hardware shows that the latter is original to the instrument, thereby making the guitar antique.
To identify the type and manufacturer of the guitar, browse through books on antique guitars such as the "Official Vintage Guitar Magazine Price Guide" available in libraries or online. In addition, check the websites pertaining to antique guitars. These resources contain photos and illustrations of different types of guitars, and provide information about the model and make of the instrument.