Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Overall, Medea doesn't exactly paint a pretty picture of humanity. For one, its two main characters don't have too much in the way of positive qualities. Jason forsakes his wife, Medea. Medea kills their children. Neither one of these people could be said to be particularly sympathetic. On top of that, neither one learns anything by the end of the play. By the conclusion, they're both still convinced that they've each done the right thing, and that the other is to blame.
The play also seems to take a pretty cynical view of love. Over and over again, love is a force of destruction not joy. Medea and Jason's relationship began with love, and, well, we all know the marriage didn't turn out particularly well. Even Creon's poignant display of love for his daughter causes his destruction; he throws himself on her flaming corpse, and he too is consumed.
Of course, the play is not without sympathy. Most of this comes through the Chorus. They express a great deal of sympathy for Medea's plight at the beginning of the play. Beyond that, Euripides seems to use them as a mouthpiece for his views on the mistreatment of women in Athenian society. The Chorus, along with every other character, also expresses sympathy for Medea and Jason's poor doomed children.