"If this was a replica, how can i determine this by just looking at it?"
For one thing, a genuine $20 gold coin will be heavier than a silver dollar.
Strange things have happened in numismatics; old attics, libraries, old collections, and such are the kinds of places from which rarities surface. That said, if your Baldwin $20 does not seem to compete well against a U.S. $20 gold coin, then it is most likely one of many knock-offs in base metal.
Also, as a footnote, pioneer gold is an area of U.S. numismatics with a lot of "problems" that have caught experts. The Red Book has seen some of these come and go from its pages as their authenticity was touted and doubted.