One last point, original paintings for magazine covers, etc are usually simple to authenticate. The magazines or cards images are direct reprints of the paintings and you can simply see if graphics they match up. In cases a brush stroke in the card will match the stroke in the card. And if someone tried to paint a copy of a Sports Illustrated painting, it will be noticeable different. Modern card artists like Dick Perez specializes in watercolors and there's no way you do the exact same watercolor twice as the watery paint has a mind of its own.
Looking at all the unique details below, you can see how a collector can personally verify a painting as being the original art for the below card. There's no way the minute details could be duplicated in a second painting, even if made the same artist. The only other thing the collector would have to be sure of is the item is a painting, not a digital copy or Xerox (hint, any print reprint would have the tell tale color dot pattern). In the case of an acrylic or oil painting, the collector could see and feel the raised texture of the brush strokes. Watercolors don't have raised brush strokes, but magnification show the waterlike brush strokes and uneven application of paint-- easily distinguished from the color dot pattern of a reprint. Much of the original art for 1950s-60s Topps and Bowman baseball and football cards were flexichromes, which were hand colored plastic film (very bright and gaudy colors typically). A verification key is flexichromes and the flexichrome process were commercially discontinued in the 1960s. A flexichrome itself is a bygone relic of the vintage Bowman and Topps era.
