Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Serious, Tragic, Sad
Milton's takes his poem very seriously; Adam and Eve's fall was, for him, one of the greatest of human tragedies [it "brought death into the world, and all our woe," (1.3)]. Satan's rebellion, his plotting of revenge, these are not laughing matters. While Milton often paints incredibly beautiful, romantic themes, he's basically never funny (with the exception of one or two very subtle fart jokes in Books 7 and 8). How could he be? He was a radical protestant, and for him the Bible was the book of books. He didn't want to make light of it.
At the same time, we can often detect a sense of tragedy in Milton's verse. The poem was originally conceived as a tragedy like something Shakespeare might have written. Somewhere along the line Milton realized that he wanted to do something different. Even though Milton re-conceptualized his poem (from tragedy to epic), he still approaches the subject matter as if it were a tragedy. At a number of points, he can't resist interjecting, saying things to the effect of "oh, would that things had been different." In Book 9, he says flat out that he must "change/ Those notes [i.e., the previous 8 books of the poem] to tragic" (9.5-6).
But even when Milton isn't being so obvious, one can always detect a sense of sadness in his voice. Yes, Eden is lovingly painted as the most beautiful place ever, but Milton always makes it clear that such a place is no more, that the only way we can access it is through poetry or the imagination. That is very sad.