Posted By:
hankronIt really ranges. Old 20th century art for cards, magazines and ads ranges can be oil, acrylic, watercolor or other paintings, sketches (ink, charoal, etc), photos, etc. There is a lot of mixed media-- meaning mixing and matching of painting, print, photo, sketches. You will often find sketches or paintings with 'paste-ups', paste-ups being cut out photos or pictures pasted to the background. Even if the original art is a photo, it is often hand embelished (adding details, background, etc). Many 1950s-60s cards and colorful advertisements were made from flexichromes-- which are small handcolored photos-- known for their super bright colors. The 1957 Topps football cards were made from flexichromes, and you can flexichromes on the market with some ease.
In general, you can look at a card or ad and make a reasonable guess as to what was the original. Some old magazine add will be an obvious reproduction of a charcoal sketch or a painting. An Exhibit Card is obviously a reproduction of a photograh. As noted, there is a lot of mixed-media, so you often can tell for sure until you see the art in person.
There are a wide range of factors relating to financial value. This includes significance (original art for Johnny Unitas's rookie card), artst (preferable signed0, size and what medium. A large detailed painting will be worth that a small quick sketch, for example. In general, collectors of the original art are looking for displayability-- collecting them as pieces of art.
For some original art, especially for magazines, the magazine or publisher's name and often the date is stamped on the back.