Your mom is super excited to hear about your A+ in science class, so she says: "Hurray! I am so proud of you!" with a smile.
not sarcastic
Jonathon is so bored in his English literature class, and he says: "I am having such a fun time" in a monotone voice.
sarcastic
Shannon is genuinely interested in learning about dinosaurs, so when you start talking to her about velociraptors, she says with a smile and a nod: "Tell me more!"
not sarcastic