A very good question! But at the moment, the answer is largely 'we don't know'.

There are some indicators that there was a large Signals Intelligence operation based in Hong Kong by US intelligence from the 1950s onwards. In 1955 there was a consulate staff of more than 150 permanent members serving the interests of around 2000 American citizens in the colony. This very much suggests that quite a lot of these officials were there operating under diplomatic covers.

Here is the George Washington Archive collection on US intelligence activity in Asia.

This is something I have on my personal research list as well.

EDIT: One thing I forgot to mention that is very important in the US intelligence operations against China is the strange case of James Angleton, head of US Counter-intelligence within the CIA. He personally thought that the Sino-Soviet split over communist doctrine and Stalin's legacy was a fabrication. This meant that much of the intelligence gathered about China and Sino-Soviet relations through the 1960s up to 1974 was considered to be an elaborate disinformation campaign, greatly weakening US intelligence operations in relation to China.

Source: C. Andrew 'For the President's Eyes Only: Secret intelligence and the American Presidency from Washington to Bush'