In "I am swimming", "swimming" is a verb. If it were an adjective, you could modify it with "very", but *"I am very swimming" is no good. In "I am confused", "confused" could be an adjective or a verb -- it may be ambiguous; I'm not sure. But at least, it can be an adjective, since we have "I am very confused". (Note that "killed" is always a participle and never an adjective, so *"He was very killed" is bad.)
We can't use predicate position as a test with coordination, but elsewhere we can devise a test based on the fact that only like categories can be conjoined. Both -ing verbs and adjectives can modify nouns, as in "the confused athlete"/"the swimming athlete", but *"the confused and swimming athlete" is pretty strange, presumably because we've coordinated different categories: adjective and verb.
(John's answer, given above as a comment, is wrong. Category is intrinsic to constituents -- function is not separable.)