Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Yes, that's right. Some people think Sophocles morally condemns Electra and Orestes' actions, while others think he's not interested in morality. Another possibility is that neither interpretation is true. Electra might be intentionally ambiguous: it neither ignores morality nor directly condemns its protagonist(s). Sophocles leaves the issue to his audience.
When you're talking about tone in a Greek play, you always wanted to consider the Chorus, which is important in determining the tone. The Chorus sympathizes largely with Electra. They lament with her when she suffers, and rejoice when she reunites with her brother and achieves her long-desired goal at the play's end. An interpretation that centers on the Chorus might condone Electra's actions. Again, you have the liberty to choose the interpretation you find most compelling, provided that you have evidence to support your claims.