I assume there's people in gov who keep up with tech progress (military leaders, DARPA scientists, etc.) so they would have a good idea of when the time for nationalization/confiscation would be right and relay this belief to the lawmakers/head of state so that they could take action.

In any case, I expect politicians to raise their interest in AI as the technology advances, and future senators may not even need a heads up to introduce legislation to make AI-researching companies state owned.