Be warned. Some spoilers are in this article for Age of Ultron and Agents of sheild. If you haven't seen either, come back later.
It was really easy to know that Avengers: AoU would start in Sokovia if you watch Marvel's Agents of S.H.E.I.L.D. at all. Specifically, the episode released on Tuesday April 28th as it directly pulled The Avengers into perspective.
Agent Hill and Director Coulson have a conversation in which Hill asks Coulson if THETA protocol is ready. Coulson says "yes. It's time to bring in The Avengers."
Now this alone wasn't enough to know about the Avengers' involvement in Sokovia. But what did help was the fact that before THETA protocol was mentioned, Coulson told Hill that he knew Loki's scepter was in Sokovia and that he was pretty sure that List was headed over there.
From past experience in the MCU we know that List and Baron Von Strucker are Hydra agents and thus enemies to the Avengers. When you put the end of the episode of Agents of shield where Raina sees "an army of metal men" you get a bigger idea of Age of Ultron. 2+2 equals awesome opening scene.
What are your thought? Let me know in the comments. Thanks for your time!