This congressional letter does a decent job at synthesizing the main logic: https://democrats-foreignaffairs.house.gov/wp-content/uploads/2026/11/letter-to-commerce-on-ai-chip-export-to-chinafinal.pdf . The main evidence is an NYT article based on anonymous sourcing about US intelligence which of course isn't the strongest. However, I do think that generally it would be challenging to have good public evidence that someone is a spy because the point of spying is to be hidden.