Oxidation flavors can take many different forms. Wet cardboard and paper are some of the more obvious and offensive ones, but it won't always present that way. In general, you'll notice a dullness of flavor, where what was once bright, fresh and clean will seem muted, dusty, and bland. It can make a beer seem more caramelly sweet, or fruity, depending on what was in the beer before it was oxidized. Some take on sherry or madeira notes. I think melanoidins and maillard products tend to get those sherry notes, while acids tend to get estery. I also tend to notice the finish getting harsher and seemingly more bitter. The color can fade or at least darken in hue. It might start getting cloudy or showing flakes suspended. There are a lot of chemical compounds in beer, and all of them will oxidize. Each reacts differently. So the art is in deducing that it has happened when it shows up in so many different forms and intensities. It's not like flipping a switch; there is a matter of degrees. For the average beer, I'd say it usually shows up as dull and more caramelly, before it starts getting obviously stale.